There is a lot of (cost)-free stuff out in the net but good content with a https://creativecommons.org/licenses/by-sa/4.0/ is harder to find. One example of such a product is the gimp magazine. Issue 6 is out since a few days and all Issues are free to download on the gimpmagazine homepage as well as free-as-in-freedom, cc-by-sa licensed.
The gimp magazine is made for graphic artists with a focus on tutorials, howtos and art made with the free/libre/opens-source program Gimp - the Gnu Image Manipulation Program. Gimp itself is a free alternative to popular non-free graphic programs such as Photoshop.
There exist not (yet) an epub version of the Gimp Magazine, but it is possible to download it as pdf or view it online in the browser thanks to the website Issue.
Despite being free, the gimp magazine is professionaly made and sells ads like any high quality (paper) magazine. While i'm personally not much of an graphic artist i enjoy browsing throug the gimp magazine a lot.
The nice british guys of the Linux voice podcast decided nearly a year ago to fund a magazine via kickstarter: The linux voice magazine. Since then, 10 Issues were published, full with storys, tutorials and news articles about free software. Subscription (online, paper or both) is not cost-free (but money well spent). However, the linux-voice makers promised to release their content after 9-months under a free cc-by-sa license. As promised, the first issue of Linux voice magazine is free licensed and free to download for everyone:
I like the Linux Voice magazine a lot and am a proud (paying) subscriber. To my great joy, the online version of the Linux voice is not only aviable in pdf format but -since the last issues- also in (DRM-free) epub format. This makes reading my favorite computer magazine on my ebook reader possible. The epub format is experimental and have some minor problems, like destroying the layout of python source code inside tutorial articles. Also graphics look naturally better in pdf (colour) than in epub (black/white). I wish more magazines would offer their content in DRM-free epub format, as it makes for a very pleasant reading experience.
The linux voice team has promised to donate half of it's profits back to free software projects and has recently started a voting project for (paying) readers to decide a selection of supported free software projects. One of the candidates able to vote on is the Gpodder Podcatching software project of (among others) Thomas Perl, who sometimes speaks on my Biertaucher-Podcast.