The Sagan Diary is a novelette set in Scalzi's Old Man's War universe and is presented as a series of diary entries by Lieutenant Jane Sagan.
I really loved Old Man's War and The Ghost Brigades, but I thought Scalzi whiffed with this one. The novellete is neither interesting as a stand-alone piece nor does it add anything substantive to the character of Jane Sagan or the series storyline. What it most felt like was Scalzi trying to write like a woman and resorting to generalities and stream of consciousness prose.
I suggest steering clear of The Sagan Diary unless you must read everything that Scalzi puts out.