This is another activity which is totally revolutionised by the internet. Within hours of listing books on Bookmooch, I had requests for some of my stock of surplus books.
There are two sorts of book swap, those where you swap directly and those which use a "currency."
Readitswapit is a direct swap site based in the UK. The drawback as I see it is that the chances of someone who wants your book having one which you require are pretty slim. I may, of course, be totally wrong.
There is a clear explanation of how this site works here.
The second model seems to work very well however. I've used two sites - the market leader is Bookmooch, with the main disadvantage being that it is much more used in the US than in UK & BookHopper, which is UK based. The eBay effect comes into play here and I have had much more success both giving and receiving books on Bookmooch.
How Bookmooch Works
You register, list the books you have to give away & you get .1 of a point for listing a book. Once you have a whole point you can request a book. It costs 1 point in your own country & 3 if it is from abroad. You may refuse any mooch. There are official acceptable reasons for this (and unacceptable ones... I'm amused that "I've lost the book" is not an acceptable reason! If I've lost it, I can't send it!), but when all is said & done, no-one can force you to send a book out.
It is a good idea to add condition notes to your book entry and I also use this to say whether I am likely to agree to send the book abroad. I have stopped sending heavy hardbacks anywhere as they cost too much to post.
You can decide whether you wish to send abroad or not by setting your account to "only to my country", "ask if not to my country" or "worldwide".
Once someone has requested a book and you have decided to send it, you pay the postage (obviously) but surface mail or 2nd class for domestic mooches is acceptable and, in return, you pay nothing for the books that come to you.
I put a large number of books on at the beginning and built up rather a large balance of points. Now I add relatively few new books to my inventory while I try to spend them.
As well as adding your inventory of spare books, it is worthwhile to add books to your wishlist. When a book is added to those available on the site, an email is sent to one of the people who has wishlisted it. That person is chosen at random. However, from the moment it is listed, anyone may mooch it so it is worth checking now and then to see whether any of your wishlisted books has been added.
I love using Bookmooch, not just because I have turned books I don't want into books which I do, but because many recipients are delighted with their books - one on Perpetual Motion went to India, Paradise Lost to Turkey & two copies of the Young Visiters to the US, all to delighted moochers.
There is a load more about Bookmooch here.