I *do* read every book I intend to use heavily from cover to cover.
But, for most books, I flip through them first, looking for what interests me, and decide how / when I'm going to read it based on my first dig.
For example, I ws just going through Gnomes & Kobolds of Tellene yesterday - and knew that there was little in the book I'd be using in my games (as I don't even have gnomes in my core campaign, and have never played a game in the setting of this book), so I was specifically searching for material I could convert to my own setting. In this case, it's a flip through the book situation instead of a true reading.
However, I've read every book I have for the campaign settings I do run - Oathbound, FR, and Arcanis to be specific. You learn a lot about the game by doing this - it's impressive how just reading through the spells in a book can give you new ideas for running bad guys or good guys or whatever.