My absolute fav option for old books, especially old encyclopaedias, and ridiculous classic from the school Mother's Day stand, is a succulent planter. But there are so many options to upcycle books that there really is no excuse for tossing them out. The first option should always be to consider who else might want the book.
If books are in reasonable condition and 'suitable', one of the best places you can take them is to your local school. Pretty much any school library, public or private will appreciate them. Child care facilities are also always grateful for books and toys. Some second hand booksellers will pay a small price for old books. Most charity organisations will accept good quality second hand books.
There are now over 1,000 street libraries in Australia and the numbers are growing every day. You can find a location close to you and give your books to that library or start one yourself.
Books can be repurposed into the most surprising things. Hardcover books can be hollowed out and made into hidey hole boxes. Old book pages can be converted into tags, used for origami, made into cards, book shelves, fans, vessels to hold utensils and much more.
Books are not easy to recycle as adhesives used in the binding prohibit normal paper recycling practices. Because of the contaminant problem, many books are not recycled in the same way as other papers, back into a lower grade version of the the paper they started as. Rather it is recycled into animal litter, insulation for housing and pulp containers. (So if your book is destined for recycle bin, don't feel guilty about turning it into a succulent planter or origami.)
Consider purchasing an e-reader or using your ipad. You can store thousands of books on your ipad and it weighs next to nothing. Electronic devices use no paper, create little to no landfill and save you money in the long-term.