While I love our ebook for US government, this year I have been particularly bothered by Pearson's whose servers have been closed at times during the school day. That being said, I strongly believe the textbook should just be one source of information and not the only place where students learn. Indeed, if I were a textbook executive I would be scared to death of sites such as CitizenU which has all the videos and short readings to replace the traditional government textbooks. It even includes thoughtful AP type questions. In fact the two founders, Andrew Conneen and Dan Larsen are both question leaders at the AP US Government and AP Comparative readings so really know their topics. Above is one of their videos.
There is also a group I used to work with in Philadelphia which has a great standard government textbook which you can find here.