With some non-relevant exceptions, copyright does not attach to copyrightable works created by federal government employees when acting within the scope of their official duties. 17 USC 105.
So, no copyright attaches to the text of speeches written by a President (while President) or written for a President if the speech was written by a federal government employee. Under those circumstances you're free to reproduce the speech in a book.
Answered by Daniel Ballard on 06.20.09.