Amazon is best known for books, although it does now sell a wealth of other things directly and on behalf of other retailers. Amazon sales are post free on orders over £15.
Good range of books and music, mostly at list prices but some discounts. Free standard UK delivery on orders over £20 otherwise from £2.
Award-winning 'EyeWitness' travel books. Also
a wide range of general and children's books. Free delivery on orders
over £25. 
16,000 travel books, guides, maps and phrase books at up to 30% off normal prices. Post free over £5.
Frequent offers on popular books. Have your order delivered to a store for free or sent to your home, free over £25, otherwise £3The Book People have agents who go to workplaces to sell a selection of books. On their web site you have a wider choice. They always have a huge selection of children's books and bargain sets.
The Book People sell a narrow selection of books but at massively reduced prices - typically 75% below retail. Standard delivery is £3.50 to the UK mainland but free if you spend more than £25. Similar bargains on a selected range of children's books come from Red House books. They offer free UK delivery on everything and some information pages on their web site that keep you up to date with what is happening on the children's book scene.
Pickabook offer a more traditional selection of the sort of books you will find in High Street bookshops, but at a discount that is typically between 16% and 33% off the publisher's price. These are not out-of-print books. They are current, latest editions at heavily discounted prices. From a stock of over five million books, the one you want is probably there. Delivery is free on orders over £15. For smaller orders, charges start at £2.50 for one book, £2.95 for two, £3.35 for 3 or more up to 20kgs.
Browseforbooks are very similar to Pickabook, with over 2million titles in stock including professional and technical books as well as publications of more general interest. Their prices are very close to those of Pickabook but their UK delivery prices are higher at £2.75 for the first book and 59p for each additional one. On big orders there is a flat-rate UK delivery charge of £6.99.
It is worth checking both Browseforbooks and Pickabook because sometimes one is cheaper and sometimes the other. Including delivery Pickabook generally comes out best - just.
Penguin sell their entire range online, which includes Puffin books, Ladybird children's books and Rough Guide travel books, but do check the prices on Pickabook as well, as they are usually cheaper.
Waterstones, the famous High Street book shop has a massive range of all kinds of books and they deliver free to the UK if your order is over £15. Many of their books are not as cheap as Pickabook. However, they frequently have huge offers on current best sellers, some a low as 50% off the publisher's price. Waterstones have special offers of the week, which change every Thursday morning. You can see the latest offers here.
Blackwell are particularly strong on professional books, such as medical, legal and scientific textbooks. They also have a wide range of printed music. In addition to these specialties, they do stock the traditional range of books of all natures, but their prices are not usually as competitive as browseforbooks and pickabook. The delivery cost within the UK is £2 for the first item, plus 50p per additional item, but any order over £20 is delivered free.
Amazon
are possibly the best known of the internet booksellers. They have some exceptional offers on some books, including bestsellers. Their range is also huge so you will probably find what you want here, although some books are cheaper at Pickabook and Browseforbooks, so it is worth trying them all for the best deal. Amazon do not charge for delivery if your total order, for books and anything else they sell, is worth £15 or more.
Enter title, author, or other search term
Abebooks is different! It is not a bookshop as such, but a directory of books for sale by other booksellers in the UK and abroad. You can search for new, out-of-print, secondhand and any other kind of book - even first editions. You choose your book on the Abebooks site and pay them. They then arrange for the bookshop that has the book to send it direct to you. Some of the books are delivered free. Others have delivery charges set by the book seller. You can click the logo in the box in the column on the right to go to the Abebooks site, or enter your search term in the box on the left.
If you want travel books, maps etc you have plenty of choice. You can buy from the web sites of the publishers themselves, such as Dorling Kindersley and Lonely Planet, or you can go to a specialist travel publication website, like JumboGo, who say they have 16,000 travel-related publications in stock. Not only that, they sell at below the prices the publishers themselves charge. Pickabook also have a huge selection of travel books, which are normally sold around a third lower than the price on the cover.
Overall, we think Pickabook is likely to be cheapest, but it may be worth paying a little extra from JumboGo to get their free postage on orders over £5. Also try JumboGo if Pickabook have not got what you want and the publishers themselves only if you draw a blank everywhere else.
Books Direct is a bit different. They are a book club with attractive introductory offers, in return for which you enter a commitment to buy a specified number of books in a year, discounted from the publisher's price. This can be as low as 2, but is usually more.