I agree with the poster who said that she has found more useful information online than in books. I've been checking out books in the lending library at a local school for children with disabilities, and while there is some useful information out there in books, I've found tons more online, and it's free! If you have a lending library near you or can find books about Asperger's at the library, by all means, read them, but buying them seems like a waste to me when there's a wealth of information (did I mention that this information is pretty much free?) online.
One book that I will recommend to you that I'm reading now is Born on a Blue Day. It was written by Daniel Tammet, a man who has been diagnosed with Asperger's, synaesthesia, and savant syndrome. It could provide a lot of answers for you as to why your child thinks the way he does. He goes into the savant syndrome and synaesthesia at times, but mostly he talks about being shy, not being able to relate to his peers, having troubles in school, that sort of thing. If it doesn't do much for you in that regard, it's a very good read nonetheless, and it's a popular book, so it'll most likely be at your local library. I have read quite a few books on autism and Asperger's in particular and I haven't found a book that really captured what it is to have the syndrome quite like this book has. He really makes it easy for people who don't relate to how he feels to understand the way he sees the world. I found myself nodding and agreeing with a lot of what he said throughout the book.
I haven't read any of the books by Temple Grandin, and I haven't read more than one by Jerry Newport (both also autistic) but they might provide some insight, as well. I wouldn't bother with the ones written by "experts" who don't have the syndrome myself because they won't portray what it's truly like to have AS, because, well, they don't have it. Any of the books I've read that are written by "experts" just gave me a lot more questions than they've answered for me, and I just don't think most of the ones I've read were worth reading, personally. They tend to pathologize autism rather than Daniel Tammet's approach, where he just tells you "this is what I experience and this is how I try to cope with it", and the former approach makes me feel awkward and almost dirty after I finish the book, like I need a shower or something. Anyhow, I've written a novel here myself, but here's the summary: try to focus on books that were written by people who are autistic rather than "experts" who are just observers.