It’s been a long time since I studied discrete mathematics formally, as an undergraduate. I haven’t yet used this text.
That said, I’m keen on finding amongst all the so-called “open education resources” (OER) those that are truly free and open, which is to say, licensed in a way that encourages and enables widespread use and adaptation as close to the four freedoms of free software as one can make a work, and without tethering use to proprietary platforms or formats. OER should be “open” all the way down, not just free-as-in-pizza to use. (cf “it’s about freedom, not price”). A lot of OER discussion that I’ve seen emphasizes price, many prominent platforms (including one funded by the US National Science Foundation [NSF]) run atop proprietary platforms, and many nominally OER works are only available with non-commercial or no-derivative terms.
In that light, when having discussions recently with some people about OER, I’ve used this discrete mathematics text from Levin as an example of OER done right.
Its source is available in a reasonably-widely-used, non-proprietary markup format (LaTeX); is cloneable using a widely-used, non-proprietary format and protocol (git) which is publically hosted (on GitHub, but could be put up elsewhere easily enough); and is licensed in a way that allows distribution verbatim and of derivative works (CC-BY-SA).
What’s more, this is in a field in which the main textbook I’ve seen being used recently at two of the more prominent universities near to me is expensive and has at least this year been availabe even in hardcopy only as a rental. That’s right, one cannot even buy and keep the physical textbook from either of the campus bookstores!