I've heard legends. Apparently, if you reach some stage in your career (they won't tell me which one), you get sent free books. All the time. Without you having to ask. Apparently it becomes rather annoying.
For those of us still at the 'I spend HOW MUCH on Amazon last month!?!?' stage of things, we would welcome the 'annoyance'. Fortunately, there are still a few ways of getting free books even at an earlier stage of one's career. And no, I don't mean writing one (they don't usually give you many anyway).
1. Write reviews.
This is perhaps the most obvious and may be slightly onerous. Of course, at the end you not only get a free book, but an extra line on your CV. Different journals do it differently. Some will not accept *any* unsolicited reviews, and they list the books they are currently looking to have reviewed. Others only work by unsolicited submission. So you have to check. And publishers work differently too. Some require that the journal get in touch to ask for the book, others have an online form for you to fill out, and some just send them directly to the journals. But it usually doesn't take that long to find out. You can also try writing reviews for your institution, if they have a blog, or create your own (though of course this won't be as 'esteemed' as an established academic journal). If you're going to have to read the book for a lit review or paper anyway, might as well get some extra credit for it, and the book for free, by doing up a review.
2. Make other people write reviews.
By this I mean, start your own journal. And make sure that when people line up to ask to write reviews, you're the one asking for them from the publishers. Before long, publishers will start sending books before anyone has asked to review them. Those that you can't find a reviewer for, you get to keep!
3. Order exam copies.
This really only applies if you're teaching on a course, but I've known too many people to fork over the money for teaching texts, when they can get them for free. For most major academic publishers you can request exam copies of standard teaching texts online, either through an online form or via email. They'll often ask you to fill in a survey or something afterwards, but barring that, it's completely free.
4. The discards pile.
When senior academics retire, move, switch offices, or attempt to tidy up, they usually get rid of a BUNCH of books they just can't be bothered to take with them. You'll often see them sitting outside their office in a cardboard box with the words 'FREE' scrawled on the back of an undergrad's late paper. Keep an eye out for such things, especially when it comes to those academics closely related to you in topic and interest. It's a goldmine, especially because they've often highlighted the most important bits for you. If you're really in good with someone, maybe ask to have a first pass at it before it makes it out into the hall, so you can be sure not to be left with the scraps.
5. Publishers' surveys.
If you get on a mailing list for your favourite publishers (which I suggest you do, just to keep on top of things), occasionally they'll ask for opinions through online surveys. The reward? A book of your choice.
6. Book launches and conferences.
We all want to be helpful out of the goodness of our little hearts, but sometimes we can have our own motives. Planning book launches, events and conferences can be a great way to get a hold of books, as publishers will often send a handful, without expecting you to send them back. And those in charge always get first dibs. If an academic is planning a book launch, offer to help out, the reward is almost always their shiny new book, with a personal autograph just for you.