I just noticed, that today there is a Back to School sale at Packt Publishing: each LaTeX ebook (and other ebooks) are available for just $10 today. You can also bundle two LaTeX books and a third book for $25 all-together.

Check out the details of the books, on the publisher’s page or for example at LaTeX-Cookbook.net.

That’s the link to the sale: https://www.packtpub.com/all?search=latex