I have personally not readed any of these books but i would really trust Jan Ove Reins advice of which books are good or not.
The only advice i could give about these books was the on written by Gary Polis, but i dont know if theres any identification keys in it.
Anyway, here´s a link to his page: http://www.ub.ntnu.no/scorpion-files/litterature.htm
He do have a long list of species on his page but there is not a description of all species.
Enjoy this great page.
If you need to have a scorp identified, have you tried posting on the scorpionfans yahoo group? Many scorpion taxonomists post on the list and I'm sure that they'd be happy to help you identify a species if you need to. There's a link to it in the "newsgroups" section of my links page. There's also a link to "The Scorpion Files" Great site, but I don't know if they have any keys up on it.
As far as actual identification keys, the Polis book (Scorpion Biology) has ID keys down to genus. The only catch is that it is a true biological identification key and thus is largely useless on a moving specimen. Your best bet is the scorpion files gallery -- at least if you want to narrow it down or eliminate a few suspects.
Not that I wouldn't recommend reading the Polis book. I actualy just got its "sequel" of sorts entitled "Scorpion Biology and Research" co-edited by Brownell and Polis (pre-mortem, of course). I haven't started reading it yet, though, so I don't know if it is as good.
You might want to try "Scorpions: a Complete Pet Owners Guide" by Manny Rubio. Not a field guide, but it does have a lot of pictures and descriptions of scorpions popular in the pet trade. Probably the best hobbyist-oriented book out right now, at least until Gaban's book is published.
Dave Gaban is an expert on scorpion care and writes regular features on different scorpions in the ATS Forum mag. He has apparently written a book but TFH is sitting on the manuscript...figures, TFH doesn't want to ruin their reputation by publishing a book that's actually good!
You might also want to subscribe to the scorpionfans yahoo group.