There is not currently a good way to search across all electronic books; however, each of these links also provides a search feature to try and locate specific books on your topic. Keep in mind that most free full-text book services cannot contain copyrighted material, so they are usually limited to books published pre-1923.
Iowa State University is a member of the Digital Commons Network - the collection of institutions making use of the bepress Digital Repository platform. This resource provides a way for anybody to easily find Open Access materials (papers, articles, theses, etc.) that all member institutions have provided.