Massumi's user's guide is a wonderful little book, but unfortunately is not a book about Deleuze and Guattari. At the outset, one initially thinks that Massumi will be giving a close reading of _A Thousand Plateaus_, but quickly finds that the text is a patchwork pieced together out of Deleuze's various writings. For instance, the first part of Massumi's book, entitled "Force" discusses D&G in the context of the Plateau in ATP entitled the "Geology of Morals", but greatly broadens this discussion by interpreting D&G's appropriation of Hjelmslev's semiotics in the context of the reading of force Deleuze gives in _Nietzsche and Philosophy_. Now, not only does Massumi severely simply D&G's appropriation of Hjelmslev, but he does the same with Deleuze's account of force in Nietzsche. Moreover, in Deleuze's own independent philosophical works and in the context of his work with Guattari, Deleuze never makes use of the concept of force. Now, in and of itself this is not a bad thing and Massumi ends up producing a very useful model of analysis, but it's questionable as to whether such a reading really helps the reader penetrate what D&G are up to in ATP.
It seems to me that this sort of strategy is symptomatic of a lot of works on both Deleuze and Deleuze's work with Guattari. No one would deny that the works with Guattari and Deleuze's works "written in his own name" are exceedingly difficult and require a lot of work to unlock, and that as a rule his writings in the history of philosophy are remarkably clear. As a result, there seems to be a refusal to read the independent works on their own terms and a tendency to attempt to reduce them to the historical writings. While I would be the last to claim that the histories are to be ignored, it is nonetheless the case that the use of them ought to center around demonstrating how they converge with the independent works, how Deleuze rethinks their problematics, and where Deleuze diverges from them.
It is also likely that much of this textual practice comes from the latent imperative in Deleuze's philosophy to create. This has to do with Deleuze's textual strategy of "getting behind the author and creating a monsterous offspring." As a result, those that write on Deleuze simultaneously experience the necessity of merely doing commentary on what he said in order to show how it belongs to a philosophical tradition and problematic, while nonetheless being forced to remain silent on what he said. What seems to be forgotten are Deleuze's words immediately following his pronouncement of getting behind the author, where he claims that the only rule is that the author himself must be shown to have said it. Moreover, much of the "creating" that goes on in the name of Deleuze and Guattari comes to look like an arbitrary activity based on the will of the author, rather than an expression of the impersonal and necessary that D&G were always quick to emphasize. In other words, sometimes the greatest usefulness in writing about a text consists in getting clear on what that text actually says in its own terms.
Massumi's book can be highly illuminating and is a great and exciting read, but is not necessarily the best source for coming to understand Deleuze and Guattari's difficult texts. One would do much better to first read something like Eugine Holland's book if their seeking to get an accurate picture of what's going on in Deleuze and Guattari.