zorlag
It's true that there is two books about rogues already: S&S and T&T...
However, both of these were dissapointing in one way or the other:
Song and Silence has pretty good selection of feats (meaning that there is something useful), mediocore prestige classes (Only ones that I have some use for are Spymaster, Thief-acrobat and Virtuoso), poor equipment section (A lot of about instruments, too much IMO and only few interesting items), quite a lot of playing tips (for rogues especially) and some spells, most of them so-so (like Fanfare no-save stunning anyone?). Oh and lets not forget garrotte rules, which are good with one exception (Locking garotte is kinda, ya know, 'broken')
Traps and Treachery has poor selection of prestige classes (mechanically broken, ideas are actually better than in the S&S), lackluster feat selection (only 2 or 3 actually useful, couple are broken <Improved Sneak Attack, Quickstrike>), nice equipment section (that miniature golem is hilarious!) and good bit of info on poisons (better that S&S) and of course a lot of traps (some very good ideas) and premade puzzles (useful in a pinch, I'm sure).
I have both books and I still feel that even now I could use more QUALITY material on the subject. However, I have enough information how to build thieves guild, unless Mongoose comes up with detailed costs of operating such guild...New feats would be very nice, ones that are useful (not those +2 to two skills, I already have modified skill focus to give +2 to 2 skills or +3 to one, whichever the player desires). Some spells for assassin would be nice as well (both SS and TT had some, but not enough).
Oh and better prestige classes, both of the aforementioned books are quite awful in that respect. And much more new alchemy items (I'm a fan of alchemists fire

)
Z.