You can totally change out what you are reading for what you want to be reading with free books online. There are pluses and minuses to all the different book swapping sites so you want to get the right fit for your situation and the items you already own.
Let the swapping site do most of the work for you. You can sign up to receive emails to let you know when a new trade becomes available or get alerts when it's time to put a book in the mail. Some sites let you freeze your account if you don't feel like trading or are going out of town. Most trading services will automatically pair you up with other traders.
Find a site that is popular. You have a variety of book swapping sites to choose from. A littler site may allow you to interact with people you already know online but the site also needs to have solid terms of service so that you don't get cheated. A larger site will also have a wider variety of items to choose from.
Decide how you want to ship. Some services have printable postage so you may be able to skip the post office altogether. You also need to figure out whether you'll be able to send items out the next day or if you need several days to send an item because this will determine the best trading site for you.
A book trading site should have penalties in place. If a site has clear cut rules then it will cut down on cheaters. The site should offer a refund of credits so that you don't get left empty handed when dealing with cheaters. You can find a company that has personality but still works well.
Find a company that is actually free to deal with. Make sure that you are trading books instead of just getting a good deal on books. This is a way to get free books that doesn't involve you purchasing any new ones to get your freebies. When you swap books you give away the item on your shelf and fill up your bookshelf with other people's cast offs.