$39.99 for a 44 paged PDF? On Amazon.com I can buy a hardcopy version of Compilers: Principles, Techniques, and Tools (2nd Edition) for $86.29 and its got (according to amazon) 1000 pages.
I have a few reams of paper (about 2000 sheets) you can buy for $40 if you want. That's even better value than the book you suggest.
Depends on your definition of value. Aho Sethi Ullman will teach you rather more about compilation of programming languages than 2000 sheets of paper ;-)
It will go up to $49.99 after 500 copies will be sold (ok, another marketing tool I guess)
Yeah, as if he's going to sell 500 copies at $39.99.