I buy ebooks straight from publishers like Nostarch and Leanpub. (In fact, I have an older edition of this book). There are a few books that are sold directly by the authors too. All of them DRM-free.
I actively avoid publishers and sellers who don't respect me as a consumer/reader. People need to start demanding better deals, or else we'll end up with monopolies that won't think twice about deleting books in your custody that you purchased from them.
Got any notable suggestions from Leanpub?
The Kubernetes Book, by Nigel Poulton. A very detailed and well explained book that is updated every year and you get them all for a one-time purchase.
Build Your Own Lisp, by Daniel Holden. This is an often cited classic with a lot of precious concepts explained in detail. It can be read for free online. But this is an opportunity to reward them for it. (Yes, I do own a purchased copy).
Ansible for DevOps by Jeff Geerling. Already mentioned by the sibling commenter.
There a lot more on my wishlist. All of them are competitively priced at 10 to 15$ each.
Jeff Geerling's classic, "Ansible for DevOps", since naturally you'll be configuring PF on lots of servers.
[1] https://leanpub.com/ansible-for-devops
Yarr, that do be a problem matie.
In all seriousness, how has DRM not yet been recognized as the failure it is?