Finding books at reasonable prices
Most of the time you are studying, it is possible to go without purchasing a book. After all, that's what the university library is for. However, sometimes the library does not have a particular book you need, or you are looking for a popular title which is consistantly borrowed, or you can borrow only for a short period of time (even hours). Hence you need to purchase a book. The main problem with academic books is the cost: they are often prohibitingly expensive. Here are few things that I have learned through a number of years being a student. What I write applies to the UK, and especially London.
Option 1: not buying at all
There are other places than your departmental, faculty or university libraries to find and read books for free. They are:
1) public libraries and
2) bookshops.
It is worth obtaining a pass for your local public library. If a particular book is not availabe, it is always possible to suggest it to a librarian who may (or may not) purchase and put the book on hold for you. The downside is the length of time it takes for the book to be processed, so not the ideal solution if you require a book urgently. If a book has been published recently and you need to look up a particular chapter or two, the easiest thing to do is to pop into one of the high street book chains. Borders / Books etc. are usually the most congenial since they have have a cafe (i.e. caffeine, chairs and tables) in their shops. One thing though: do not treat books as if they are yours - someone else might want to read / purchase them and it's unfair if the books are damaged.
Also, it is worth exploring if your institution subscribes to e-book schemes offered by many publishers. An example is www.oxfordscholarship.com. If you are looking for a quote, try print.google.com.
Option 2: second-hand books
For academic purposes, a book does not have to be new: what you want is a reasonably clean copy. You could go:
1) to a second-hand bookshop nearby or
2) on-line.
Most university towns will have a second-hand bookshop or two. Academic bookshops such as Blackwell's and some branches of Waterstone's have second-hand book departments. Such shops will know the demand and do a brisk trade in textbooks or books which are on the reading list. It will be cheaper but not markedly so compared to the new copies. The best place for bargains is charity shops, which offer books at consistently lower prices compared to bookshops proper. Visit them just after the end of the academic year in the summer. Books are sold, sometimes at ridiculously low prices, on eBay but it is difficult to find the exact book you want - give it a go if you have 30 seconds to spare. You could also try to buy a copy off from someone from the year above before the end of the academic year. The risk of these methods is that you might not find the book you need, since everything depends on what stock they have. The surer way of finding a second-hand book is to go on-line and try the 3As (Abeboooks, Alibris, Amazon). Alibris is not particularly strong in the UK so it is the least likely place to find a bargain. Amazon Marketplace is pretty decent but you have to be careful with some of the operators there - you will need to check that they are selling the books they are claiming to sell (right edition, hard-back or paperback etc.). In my experience, Abebooks is the most reliable of the three and often the cheapest source of second-hand books.
Option 3: new books
Why not splash out on a new book? Sometimes it is unavoidable: an important book has been published and you defintely want a copy. High street bookshops often have discount days for students at the beginning of the term and that could make it cheaper than their on-line rivals. Generally speaking, though, the cheapest offers for new books are found on-line: Amazon is probably the easiest to use (and why not buy a new fridge to go with your new book), though Abebooks might be worth checking too. There are specialist bookshops such as www.historybookshop.com which might beat the price of the bigger competitors. Also rummaging the remaindered sections and bookshops specialising on remainders can be rewarding. Judd Books, the Waterstone's Gower Street branch in central London and Unsworth opposite the British Library are some of the best.
Compare prices
Whether you are buying a book new or second-hand, compare the prices from different suppliers by using www.bookfinder.com. As mentioned earlier, sometimes there are smaller or specialised booksellers who offer better bargains than the big companies.