Authors and publishers make ebooks free for short windows to gain readers and reviews. Once you have downloaded one during the free window it stays in your library.
Amazon Kindle Direct Publishing
Self-published authors in KDP Select can set their ebook to $0 for short promotions. Books 'bought' at $0 stay in your Kindle library permanently.
BookBub emails readers curated ebook deals, including books temporarily priced at $0, in genres they pick. It links out to retailers such as Kindle, Apple Books and Nook.
Written Word Media
Freebooksy sends a daily newsletter of free ebooks in your chosen genres, many of them authors' limited-time free promotions.
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