Here is the best method I have found to acquire lvl specific recipe books;
You'll need the adventure pack Splitpaw Saga since the runs take place there.
The trick is that the "upper tunnels" are an instanced run that scale to your level. So, if you go in there with a lvl 26 character, the mobs will drop lvl 26 loot.
A solo run through the instance takes about an hour and yields an average of about a dozen chest drops. About a third of these will be adept 1 spell drops, a third will be recipe books and a third will be armor or jewelery drops.
With 3 or 4 recipe books an hour, it shouldn't take you too long to get the one you're looking for. And as a bonus, you'll pick up plenty of other loot for your effort.
Don't forget to turn off adventure XP if you're anywhere near a level ding since you want to stay 26 till you get what you need!
Back when I was leveling my warden, I spent a lot of time down in the tunnels... seems like any time I had an hour to spare, there I was!!! Good luck!
Edited, Feb 8th 2007 11:19am by OldBlueDragon