The rules are still a little hazy, and the plans to get it seem to be somewhere between experiment and superstition: we know for sure that you need to have 390 First Aid already to get the book to drop (though you need 400 to actually use it), and we know that it can drop from almost anywhere in the world. But apparently there are a few places where it drops more often: in Zul'Drak, sometimes in Sholazar Basin, and sometimes in dungeons. Other than that, everything else is just rumors -- I haven't gotten the book yet, and I've been 400 since I had enough Frostweave to level up.
Very strange, and strange choice by Blizzard to do it this way in the first place. It also appears that a stack of Heavy Frostweave Bandages sells to vendors for only 5g, so the days of nabbing lower-than-3g stacks of cloth on the AH and selling them to vendors for a profit may be over. Which is fine, because your guild's tailor could probably use it anyway. But I am bummed I haven't see the manual drop yet -- the better my gear gets, the less the non-heavy bandages do for me.
Update: Yes of course they can't be on the AH, because as you can see in the screenshot above, the manual is BoP. Never mind. Thanks, commenters.