To celebrate 80 years of Penguin, Penguin are selling Shorts ('Little Black Classics') for 80p available at Waterstones Amazon & more. These are great little gifts and good way of being introduced to an author.
These ebooks are out of copyright, the only downside is the quality. The editing, formatting and standard of English may be a bit 'off'.
Project Guttenberg is the original source of free books.
Open Library provided by the Internet Archive, worth a browse for lesser known works.
Internet Archive provides a comprehensive selection of fiction and non-fiction titles. Internet Archive also contains sub collections, such as Librivox.
Radio 4 have serialised readings available for free, but for a limited time. Classics include Benedict Cumberbatch reading Kafka and the adaptation of Good Omens. "Books at Bedtime" and "Book of the Week" are worth a look at, they provide recently published (but abridged) works so consider it a taster session. More from Radio 4 is available here and here.
PaperbackThe Book People is an online company that buys books from publishers on the condition that unsold books not returned. This means that they buy the books cheaper, and with low overheads, are able to offer bargain prices. Offers and discounts are available throughout the year.
The Works are a high-street discount book store; they also sell eBooks and audiobooks. Offers and discounts are available throughout the year.
eBooksBookBub is an 'alert system' for discounts on mostly self-published books. Sign up for a daily newsletter and choose your categories.
Kind of Book is like BookBub but it also is very reliable for Bestsellers.
Kindle Daily Deals are discounts organised between Amazon and the publishers. Sometimes, there are some real bargains to be had e.g. Slaughterhouse Five by Kurt Vonnegut, so it is worth checking daily. They also do a books at a discount for that month available here.
The Kindle Store also has hidden discounts: books prices regularly fluctuate and some come down to a 'sale' price without this price being advertised. It might be worth having a browse through Amazon's store every once in a while for hidden discounts; there may be a week when a book is £5.99 and next week it is £1.99.
HaperCollins have monthly deals and 'Selected eBook offers'.
Audible Daily Deal have started an equivalent of Kindle Daily Deals and, as far as I know, do not require a membership to be purchased. Today (21/2/15) William Golding's Lord of the Flies is on offer for £1.99, quite a deal.
Listening Books is a charity-based audiobook service, with Stephen Fry as a patron. Whilst not free, memberships from £20 a year it is far cheaper than Audible (£7.99 p/m) and may be worth a subscription.
ReaditSwapit uses a system similar to eBay. You list your books and can request to swap books with others, all you pay is postage. However, be careful: make sure you read all the reviews before you request/accept swaps.
Haper Collins giveaway started after a move to smaller offices left hundreds of books without a home. Three to five books are up for grabs every week. It is also worth following publishers on Twitter and Facebook for latest offers.
GoodReads also do giveaways.
Many bookstores now have a loyalty card like Boots, where you get points for every £1 spent. Every point is a penny and you can save several pounds worth. These stores are Waterstones (they also do a student card where for every £1 you get 10 points) and Foyles (aka 'Foyalty').
Check out your local independent stores too, many do a stamp card (e.g. for £10 you get a stamp, and 10 stamps is a £10 gift voucher).
Any suggestions or feedback, please comment below.