In some cases very, but in others not so much. Some of them have Patreon accounts or other ways to accept payment, but in many cases you’ll be doing good just to find a working email address for them. Ideally though I’d prefer to just pay for the books outright rather than trying to do some kind of grey area thing where I’d pirate the book and then donate the cost to them (if for no other reason than it causes tax headaches for everyone involved).