John Scalzi is a one (or three) hit wonder. The first three books of the "Old Man's War" series are the only books of his worth reading. Don't waste your time with Zoe's War, there isn't anything of substance in that book. The following books in the series just seem like Scalzi trying to milk the success of Old Man's War. There isn't anything new or interesting.

the collapsing empire books are excellent, and I would recommend them to most SF fans. but even other than that I've found scalzi consistently entertaining - not "omg this was a brilliant book" but fun books that I was happy to have read and kept me interested in reading the next one he wrote.