A comprehensive introduction to DMR is available to hams at no cost, thanks to the efforts of John Burningham <W2XAB>. His “Amateur Radio Guide to Digital Mobile Radio” won the 2016 Technical Achievement Award at Dayton Hamvention and the second edition was published in 2019. Even if you think you know how DMR works, this guide is full of useful information.
If your view of DMR is limited to the perspective of Pi-Star and Brandmeister or TGIF, this free book tells the rest of the story in its 27 pages. It is an easy read and will enhance your DMR knowledge.