There are very few academic books I've read that weren't letdowns: Archaeologies of the Future by Fredric Jameson was one, Chaosmos by Philip Kuberski was another. I'd add Todd May's Gilles Deleuze: An Introduction to that list. This might be the most aptly titled book I've ever read.
I'm usually pretty wary of books about philosophers that contain the word "introduction" in the title, because usually these works attempt an introduction of a specific philosopher: to reduce a complex system to a few buzzwords and concepts, to show what the writer means, etc. In May's case, and I'm not even sure he's aware of it, he's offering an introduction to Deleuze. Rather than trying to write a be-all-end-all account of Deleuze, a book you can read instead of Deleuze, May provides some of the philosophical background one needs to read Deleuze. While this may not seem all that interesting, Ian Buchanan, in his book Deleuzism, says that one of the problems with Deleuze is that one, more or less, has to be a Deleuzian in order to read Deleuze.
In Gille Deleuze: An Introduction, May helps short-circuit some of these problems, by injecting Deleuze back into some of the philosophical debates in which he's participating and, more importantly, drawing out Deleuze's readings of Nietzsche, Spinoza, and Bergson in order to offer an understanding of some of Deleuze's background noise. Moreover, the chapter concerned with the role those three figures play in Deleuze delves more into what Spinoza, Bergson, and Nietzsche are responding to, to help you cheat out of having to read them, instead of having to cheat reading Deleuze as many "introductions" like to do.
All in all, I would recommend hunting down Todd May's book if you want to start reading Deleuze and are finding yourself a little lost, although I would warn May's reading of Deleuze is very focused and people have offered other (more interesting) readings of him, so don't take this to be the last word but, instead, the first word.