Science Fiction(ish): + The Sparrow by Mary Doria Russell is very good; it's commonly referred to as "Jesuits in space" but I think of it primarily as a discussion of suffering. + A Canticle for Leibowitz is also really good; the first section is my favorite. + A Case for Conscience + Dracula (and Frankenstein, although the latter is more philosophy than religion)

Classics: + I know Graham Green is already on your list but if you haven't read The End of the Affair, that's a MUST. I absolutely love it. + Kristen Lavransdatter and Laurus are both lengthy but commonly recommended. + Anna Karenina is technically Orthodox, but so so good. + Les Miserables but I say get the abridged version. He needed an editor, and you aren't missing anything by not reading his 100+ pages on Waterloo or the French sewage system of the 1800s. + The Picture of Dorian Gray + Jane Austen's books are Anglican, but they are really interesting insights into virtue. For you, Mansfield Park might be the best fit, or Sense and Sensibility. + The Shoes of the Fisherman is one of my newer favorites for its depiction of the pope. It was also really interesting to read a fictionalized pre-Vatican II pontiff who ended up being very much like St. John Paul II.

My personal favorite: + The Chosen by Chaim Potok is my most favorite book of all time. It's Jewish, not Catholic. But the depiction of holy friendship, and in particular its discussion of silence as a language of God and of suffering is VERY good.

Side note: I've long thought that seminarians should read novels, particularly those with female protagonists, so as to help prepare for pastoral ministry. Seeing how we think would be incredibly helpful for spiritual direction specifically, but also for casual interactions that (in my diocese at least) are often so stilted with younger priests (less than 10 years of priesthood). To that end, I would recommend Jane Austen not for her plots, but for her characters. They are rich and varied. And while Anna Karenina was written by a dude, it has come to light that he widely plagiarized his wife's diary, which is why his characters are more well-rounded than the average male novelist. Reading with a desire to understand the female interior life could be greatly beneficial to your priestly ministry. I'll be praying for you!!

Not a Catholic author but he was a top medievalist with an insane level of erudition, but I would recommend Umberto Eco’s “The Name of the Rose”, which is set in a medieval Italian Benedictine monastery; it’s a historical novel which includes LOTS of accurate theological debates, it includes the Franciscan issue (spirituals vs regulars), all inside a Sherlock Holmes inspired murder mystery. It’s not an easy read but definitely worth it.

Henryk Sienkiewicz - „Quo Vadis” being an important work of his.

If you like Agatha Christie-style mysteries, you might enjoy Ronald Knox's mysteries and GK Chesterton's Father Brown Mysteries.

I love the Count of Monte Cristo. It's not explicitly Catholic but the final message is solid.

Father Walter Ciszek's "With God in Russia" rocked my world.

If you like science fiction, Gene Wolfe.

A Canticle for Liebowitz

House of Brede

You might like Georges Bernanos « The Diary of a Country Priest ».

Thomas Merton's The Seven Storey Mountain. It’s a memoir rather than a novel, but it’s one of the great works of modern Catholic literature—formative for generations of converts (myself included) and vocations, and as spiritually and intellectually compelling as a lot of Catholic fiction.

I really like the Diary of St. Faustina. But you’ve probably read it.

The Power and the Glory by Graham Greene

The Keys of the Kingdom by A. J. Cronin

Both are great novels about the priesthood and Christian persecution

Also enjoyed Dorothy Day’s The Long Loneliness

The lord of the rings was written by a catholic man and has a lot of similarities to the catholic church

The Power of Silence by Cardinal Robert Sarah

I love G. K. Chesterton - The Club of Queer Trades is great!

You already mention Lewis, but I find his sci-fi trilogy is very slept on!

This one is a little out there, pretty avant garde and not perfectly Catholic, but Septology by Jon Fosse (his other novels are great too) is what made me reconsider religion after years of not believing.

Seven parts, seven days — the seven days leading up to Christmas. Each part ends with a refrain of prayer in both Latin and English

How is Chesterton not in your list? Read almost anything of his. I would recommend his biographies on Aquinas and on Francis.

Shusaku Endo is considered the "Japanese" Graham Greene. I like both Silence and Deep River.

Balzac

Not fiction, but I'm currently reading The Reed of God by Caryll Houselander and it's very good. Really changed my understanding of Jesus's teachings

Silence by Shūsaku Endō, a novel about two Portuguese Jesuit missionaries sent to 17th century Japan and experience persecution.

Death Comes For The Archbishop by Willa Cather is a great one.

The Mote in God's Eye by Niven and Pournelle.  It's a science fiction first contact story.  Pournelle was Catholic (mostly), and the book is grappling with a lot of interesting ideas.  I don't agree with all of Pournelle's conclusions, but it's a great book.  The sequel, The Gripping Hand, is very good too.

Edit:  Also, Inferno, by Niven and Pournelle.  In it, a sci fi author dies at a convention and ends up in Hell.  He is escorted through the layers by his mysterious guide, Benny.  Again, I don't agree with all the conclusions - Niven and Pournelle create a hell which is non- permanent and can be escaped by traveling the layers and confronting your flaws, but C S Lewis relied on a similar concept in The Great Divorce, so I don't mind it so much.

Diary of a Country Priest - Georges Bernanos

Not Catholic, but because you mentioned Lewis I hope you'll take Catholic-adjacent.

John Donne's poetry is utterly sublime. I'm a Milton man myself when it comes to English poetry, but Donne has phenomenal range and incredible skill and is very much an early modern St. Augustine in the sense of being a tremendous sinner who desperately wished he could be a saint, and he poured a lot of that anguish, fear, and love out into his poems.

I'll name another one that I think is very obscure: the Pearl poet, so named because all four of his poems are found in only one ms., Cotton ms. Nero A.x. and he has no explicit identity.

You may know him by the reputation of his most famous poem, the extended narrative epic of Sir Gawain and the Green Knight (and so titled) which is an excellent poem in its own right, especially if you can force yourself to get to grips with the north-eastern old/mid English dialect in transition that it uses.

But the poem which most affected me and, alongside Donne, which I think helped lay the grounds for my conversion was his namesake, Pearl. Pearl is a poem of medium length (not an epic, but by no means just a few pages).

It's about a man whose daughter has died, his beautiful little pearl, and soon thereafter, in the midst of his grief and turmoil, she (or a woman who looks oddly like her, but no specific identity is ever given) appears to him in a dream-vision and comforts him with the truth of Christian doctrine before leading him on a stunning tour through the Heavenly Jerusalem, which to this day has profoundly altered and deepened my reading of the book of Revelation, from which the poem draws obvious influence. It's impossible, but I also felt it had strong resonances with Homer's description of the city of the Phaeacians in the Odyssey, particularly the palace of King Alcinous.

In general I would strongly urge you not to neglect poetry. Prose is all well and good, and you have listed some excellent prose authors who were a major influence on me as well. But poetry has a way of going straight to the heart that even the most beautiful prose does not, attenuated as it is by our minds into filtering and sifting its propositions.

Part of our prejudice as Catholics against poetry is that we read so much of Scripture in translation and have become blind to just how poetic is so often is. We're at risk of succumbing to the utilitarian impulses of the world if we aren't willing to make time and space for poetry in our lives.

You’ve probably read it since you like Flannery O’Connor but I’ve got to add The Moviegoer by Walker Percy

Not Catholic but close— anything by Willa Cather, particularly Death Comes for the Archbishop.

The Knot of Vipers (Le Noeud de Vipères) by François Mauriac is pretty good.

The Pillars of the Earth. It is not a very flattering look at medieval history in the church however it is extremely moving. It does a great job of showing how a priest that is a genuine servant can shape his parish for generations.

anything by Gene Wolfe.

There's a list I've been compiling based on recommendations from podcasting priests: https://www.goodreads.com/list/show/98296.Catholic_Stuff_You_Should_Know_A_Book_List

What I've read of John C Wright is pretty good, too. He's a Catholic writer, and his Countdown to the Eschaton series tackles transhumanism in a way that I haven't seen elsewhere.