A project I have really been getting into is reading (sometimes very obscure) source novels for all sorts of films, not out of any desire to do detailed comparisons between the adaptations and the originals, but just because I find this particular rabbit hole interesting. Here are a few that I have finished:

Edward Anderson, Thieves Like Us
Peter Bowman, Beach Red
Harry Brown, A Walk in the Sun
James Dickey, Deliverance
David Dodge, Plunder of the Sun
Stephen Geller, She Let Him Continue [Pretty Poison]
Davis Grubb, The Night of the Hunter
John Haase, Me and the Arch-Kook Petulia [Petulia]
MacKinlay Kantor, Glory for Me [The Best Years of Our Lives]
Thomas McGuane, The Sporting Club
Arkady and Boris Strugatsky, Roadside Picnic [Stalker]
Booth Tarkington, The Magnificent Ambersons
Michael Tolkin, The Player

And a few more in progress:

Richard Condon, The Manchurian Candidate
Edna Ferber, Come and Get It
B. Traven, The Treasure of the Sierra Madre

I have a LONG list of future possibilities.