I've just uploaded a new revision of the PDF of Agile Web Development
with Rails (2nd ed). It fixes all reported errata, and brings the content
in line with changes made to Rails since the book's release (so you'll
see that the section on Observers has changed, as has the section on
setting session options).
As usual, the update is available for free from
Enter just two of the three pieces of information to identify your PDF.
I recommend using your order number and your email address, as these tend
to be be the most consistent.