It's a few years ago since I read the book, so my memory is a bit sketchy, but it's definitely a good read, and like most of Heinleins books, pretty short, so you can read it in a few sittings.
I think there is mention somewhere in the book that there are more alien species than just the skinnies and bugs, but they never actually make an appearance.
I like the direction they took the bugs in the movie. Kind of like tyranids, they have technology, but it's all biological.
I think the movie has a lot more depth than people give it credit for. If you look at it as the propaganda of a fascist state, seen from the perspective of a "true believer" (Johnny Rico) I think you see much of the hidden depth that the movie contains that you might miss if you look at it as just another action movie with aliens.