My guess is that most of the books at manybooks.net that you can download for free are from the
Project Gutenberg archive, which has over 57,000 ebooks. The one I downloaded from manybooks was, although I noticed other books that seem to be permanent freebies either at the major retailers or at author websites. I wasn't signed up for their email, but when I joined a few minutes ago, it gave me the option to sign up for it. I'm curious, do the emails provide links to all the major retailers? My preferred format is epub, but I also buy a few from Amazon.
Here are a couple of ebook bargain newsletters signup pages for US readers.
Early Bird books: Mostly from Open Road Media, but they also advertise books from Rosetta Books and others. Daily
Riffle Books: I think mostly books from Harper Collins and is a 3-4 times a week
Bookdealio: I think this one is weekly
Hatchette Book Group: I think this only does Hatchette books, and it's sporadic, 0-3 emails a week
eReaderIQ: Daily email and also has a service to email price drops for ebooks and authors at Amazon, links only to Amazon, and many freebies are only published at Amazon
BookBub: Daily email
Goodreads used to have a daily email, but they were bought by Amazon several years ago, and when they stopped about a month ago, they referred people to the Amazon Kindle Daily Deals. I guess Amazon would rather their subsidy not provide links to their competitors. You can sign up for a daily email from Amazon for the Kindle Daily Deals, but I just use a bookmark with my browser.