One really good aspect of ebooks, especially the Kindle version, is that there are several services you can sign up to in order to obtain free or discounted books. Self-published authors sign up to such services in order to promote their works, which is good news for us readers!
Each of the services below have a free sign-up procedure, and each allows you to stipulate the kind of books you'd like to receive notifications about.
Author Marketing Club
Seems to specialise in Kindle ebooks. Some are free, some paid for.
This seems to be the sister website of Freebooksy (see below). Low-cost books, mainly $0.99. Largely KIndle, but also Apple and Nook formats.
Free and low-cost books. Unlike the other sites featured here, prices are in GBP rather than USD. Formats: Kindle, Apple, Kobo, Google Play.
Free and low-cost books. There are absolutely loads of categories, and you can choose how many books are included in your email notifications, either 12, 25 or 50. I've just changed my preferences to 12 a day because I found 25 somewhat overwhelming!
The formats seem to be Kindle and audio (for some books).
As the name implies, this site deals in free ebooks, plus one or two paid-for books per email. Formats are Kindle, Apple, Nook, and Kobo.