There are so many great, great books out there which have never yet been adapted to screen.
A Few For Starters:
The Quincunx by Charles Pallister - this is a weighty tome and a half. A rollicking epic trawl through Victorian England and a gripping thriller from start to finish. I guess I know WHY this has never made it to screen - the protagonist is a boy who grows up throughout the course of the novel (tricky to cast) and the narrative weaves through a number of expensive period locations: Victorian London - the posh and the poor (including some rather yucky scenes set in the sewers) and a grand country pile. And an enormous cast - worthy of War and Peace. All a tad pricey ... but this would be a spectacular BBC/Masterpiece Theatre TV series, believe me!
Katherine by Anya Seton - this was my mother's favourite romantic novel, but it's a lovely tale for any generation. Set in Medieval England, it charts the tale of Katherine Swyford who becomes mistress to John of Gaunt, the most powerful man in the realm. I know 20th Century Fox have optioned this book - so where's the movie? Come on guys! It's a classic.
The Hand of Ethelberta by Thomas Hardy - one of Hardy's minor novels, but a real ripping yarn with a supercool postfeminist heroine. Funny too.