Many of these character supplement books were very much hit or miss. The fighter's one was good, as was this one, but I must say that I pretty much ignored the Priest's and the Wizard's handbooks because, well, they did not add all that much to those particular character classes. Thieves are pretty cool characters in and of themselves, but the problem is that you generally play a loner. You are not good at combat and you generally hang around twiddling your thumbs unless there is a lock to pick or a trap to spring, though you do come on your own when you sneak up behind a bad guy and – STAB THEM IN THE BACK!
These books generally tried to add more life into the characters by providing kits and backgrounds to help you give more to your character than just a bunch of stats and titles, though for that to work you need players who are actually going to role play their characters as opposed to roll playing them. However, the really good players actually did not need books like this because they could create their characters and do things with their own imagination rather than relying upon a set of rules and a set of books.
Ideally, a character should only be based entirely upon the stats, and the stats dictate the character's strengths and weaknesses. However, for that to work you actually need really good, and imaginative players that are able to create a background and a series of skills and abilities based entirely upon the character's stats. However, in the real world, most people are not that gifted so end up having to fall back upon a bunch of rules to help them define their character. That is all well and good if they are wanting to role play as opposed to roll play, however, if all they want to do is to compensate for their inadequacies in real life then no amount of rule books are ever going to be able to help them.