All good suggestions, I'd add Lois Mcmaster Bujold's vorkosigan books as a fun read. Alot of them are available in the Baen Free Library if you're an E-booker: Authors - Baen Books
That's got alot of good authors: Bujold, Niven, Drake, laumer, saberhagen and others. Only one or two books by each writer but it lets you check them out handily!
I also love A. Bertram Chandler's Grimes novels
Of course, Heinlein FTW!