Q:

IS it okay to link to books you have to buy in an answer? 

I just left an answer here that suggest the use of a book that you should (although there are other ways to get a copy) buy. 
Is it okay to point people to solutions that are commercial?  Especially as I don't stand to benefit.  

A:

Well you should at least get the name of the book right, I'm sure Mr Tomlinson would be most perturbed ;) 
But generally speaking: no, answers containing nothing (or little) more than a link are not considered answers on the Stack Exchange network.  
The information that answers a question needs to be here, not on an external site (drupal.org being sort of an exception in a lot of cases), or in a book, otherwise the answer becomes useless when that site goes down/book goes out of print/etc. 
Linking to the book itself, however, is not an issue, as long as the rest of the post actually answers the question directly.