Man, lists are boring. Tell us why the books are your favourite!
Off the top of my head and in no particular order:
The Enormous Room, by e. e. cummings
A semi-fictionalised account of cummings' experiences in an internment camp in France in World War I: he was driving an ambulance on the Western Front, and basically being a bit odd and subversive as was his wont, and the French decided he was a bit too much of a suspicious character for their liking during war time, so they arrested him. In the book cummings constantly takes delight in his surroundings, even in the most abject misery, and in his fellow internees, who other people might regard as the scum of the earth. He present such an unusual and fond view of the world, even at its worst, that it's impossible not be swept up in his pure joy in life.
Just Above My Head, by James Baldwin
Baldwin is possibly my favourite novelist, and this is a big, ambitious book in which he brought together every single one of the themes he explored throughout his career: homosexuality and sexual relations, family and community, brotherhhood, the Civil Rights movement, and the performing arts. The book is almost 19th-century in its breadth and in the way Baldwin, as ever, takes a big group of characters and shows how they're bonded together, but thematically it couldn't have been written in any century other than the 20th.
The Life and Opinions of Tristram Shandy, Gentleman by Laurence Sterne
You could describe this as the ultimate post-modern novel - if it weren't for the fact that it was written in the 1700s. Sterne sets the novel up as ostensibly an autobiography of Tristram Shandy - and then in 600 or so pages barely gets beyond the title character's birth as he keeps taking asides, telling tall tales, and generally pulling the reader down figurative rabbit-warrens. Sterne's imagination and approach to the art of the novel is, at times, audacious: at one point, after the death of a character, he completely blacks out two pages "for mourning". At another he has a character swish a cane in the air - and demonstrates this by breaking up the text with a line-drawing of the shape the cane made. He cuts off characters in mid-sentence to go on thirty-page-long tangents before returning to the exact point he'd left from. It took me about fifty pages to get into the rhythm of the novel and start enjoying it; by the end I'd understood less than 50% of what I'd read, but enjoyed every moment of it. There's just not another book like it.
Ancestors, by William Maxwell
Maxwell tells the story of his family, and writes about what various of his ancestors did with their lives. Nothing remarkable about that, but over the course of the book Maxwell builds up such a strong sense of continuing and shared humanity, and the way in which we're all bound together across time and generations, that the reader can't help but be moved. Maxwell always wrote with great humanity, compassion, and wisdom, never more so than in this book.
The Encyclopedia of Superstitions, by E. and M.A. Radford, edited and revised by Christina Hole
A book that demonstrates just how many of the little, apparently meaningless gestures we make every day actually go back centuries, and were once invested with the most significant of meanings by people. Take just one example: remember the last time you went to a restaurant, and the meal came out with a little parsley for garnish? Just decoration, right? Actually, people used to think that parsley was a general cure-all for poisons, so as an act of good faith when somebody came over to your house for dinner you'd put a little sprig of parsley on top - sort of "Why would I poison you, then give you the antidote? Obviously, then, the food is safe to eat". This book is full of tiny things like that that make the human world make just a little bit more sense - while also making them seem even more bizarre than they already appear.