Set of 10 sticker books, £9.99 at the book people. Use voucher codes 204A1H to get free delivery and IS001 for 10% off, so a total of £8.99 delivered, less than £1 per sticker book!
There are two sets available - girls and boys. The link is to the girls collection but if you search "sticker" on the Book People web site you should find the boys set. I ordered both for my not quite 2 year old daughter as she loves stickers so these sets should keep her going for a while!
The DK sticker books are good ones too - I got the animal one from Amazon for her stocking and paid over £3 for one book.
The Book People are also on Quidco, though they do say you won't get the Quidco if you use codes. Still, worth a try?
Sorry, can't work out how to add a picture.