Either Roald Dahl's Matilda or Terry Pratchett's Granny Weatherwax stories. (Sorry, that's not just one book.)
I read Matilda when I was six. For months after, I spent ages trying to move things with my mind. Unsuccessfully, of course, but it taught me patience and focus (it occurs to me as I write this, it must have been an early form of gnosis!).
And then, at eleven, I found some of the Discworld books at my school library. Granny Weatherwax's "headology" - a sort of mix of pychology, expectation, theatre and bloody-minded belief - is probably the most practical and realistic and effective magic I've ever come across.
Neither book is a traditional grimoire or dusty magical tome, but they're foundational to how I actually do magic.