You're making me feel old...
I remember the whole concept of monster characters being represented with 'monster' levels from way back with three books about it for D&D (not second ed - honest to goodness D&D!). There was one of woodland creatures (sprites, unicorns, treants, centaurs, etc), one of flying critters, and one of underwater creatures. Since you couldn't really multiclass in that game, it had (I think) 10 or 20 levels for each, and one of the levels represented 'maturity' at which point you were equivalent to a monster. You could keep gaining levels becomeing a more experienced version of that monster type (keep in mind elves and dwarves where classes in this game).
All in all cool to see that come round again. IF SS has a section on creating your own monter classes from ECLs, then it'll be a great companion volume ot Race of Faerun, but if not, the two books will kind shoot each other in the foot. I was grooving on Faerun, but I think SS will get my money first.