Mary Norton's The Borrowers
Mary Norton's 1952 children's novel, The Borrowers, is the focus of Studio Ghibli's upcoming feature film, Karigurashi no Arrietty, which is set for a summer 2010 release. I have to admit that I've never read this book, but it has won many prestigous awards and is regarded as a classic. I may just have to scour the local bookstores for a copy.
The Borrowers spawned a series of books, in the tradition of children's literature, spanning through Norton's life. Interesting fact: she also wrote "Bedknobs and Broomsticks." Very interesting.
You can read the Wiki page for The Borrowers here, although I should advise you there is a plot summary. There's no indication how faithful Ghibli's adaptation will be, so newcomers might not want to spoil any surprises.