[living-0] Street smartness: Rules of thumb when buying a technology book

We have a saying in Bangla(my native language), "No one gets bankrupt for buying books". I buy a lot books. Many of the times I have found that I am paying 10-15$ more for a book and that is not good. Many of the times I have found that some the books I bought became a great teacher and companion. Other times I have found that I don't enjoy reading the book as much. Here is what I have learnt from trial and errors regarding buying books. 

I buy if

1. The technology you are willing to learn is time tested, definitely buy a good book. Examples can be algorithms, cryptography, mathematics, compilers and physics. 
2. in Quora, you hear a lot of good things about the book, in amazon book review you see lot of positive review that is an indication of that book being a good one.
3. the book is about a person you are greatly interested. For me I like Buddha, Richard Feynman, Donald Knuth, Randy Pausch (and so many other of my heroes) very much. I would not hesitate to buy a book by them. (For Buddha I would get a book that has His quote)

Before I buy

1. Verify the book's price in various book selling websites. I used to buy books only from amazon. But later I learnt about other online book sellers such as abebooks, thriftbook, alibris. I won't buy from a website that asks for my bank information and does not accept credit cards. I would rather prefer buying from a website that accepts paypal payment or at least accept credit card.

I don't buy if

1. If there is an active website that maintains that technology. An active website serving purpose of documentation for that technology is alive meaning you will get lightning fast error correction, update and improved explanations over there. Example, Hadoop, HBase, Kafka, Git etc. As soon as that technology has an update, your book might get obsolete.