I think a book being free is a good reason to start learning about something. Let's say someone wants to read something about cryptography knowing nothing about

I think a book being free is a good reason to start learning about something. Let's say someone wants to read something about cryptography knowing nothing about it. Paying $100 for a book that he doesn't know if he'll read it thoroughly or if it will be interesting seems a bit of a waste; better take a free book to get a taste of the new field, before buying another one - or donating to the author of the free book. Having said that, I noticed that the fact of paying for a book makes me more motivated to read it, while I often read couple of pages of the free books before giving up.