If I were looking at new books, I'd take a look at Track of the Wolf. They show the table of contents, so you can decide which one(s) get your interest. I'm not sure, a few of the rare volumes were recently reprinted, but I suspect #6 is available from it's original printing and overall there may be less of them out there for retail sale.
If there ever was a set of something and you have some interest, you might consider getting them all while they're easy to come by. The rook rifle books are very impressive additions to the series.