I read the books first, but that was because I had them before the comic came out. I love the books, and the comics are a great addition to them. I recommend reading the books first because the comics start at the beginning of Roland's life as a gunslinger rather than from the first book. A lot of what the first comic covers is in the 4th DT book. The DT comic wikipedia page has the correct order of the comics.
I love how the comics cover what was said in the book as well as covering missing areas of Roland's past. They also incorporate the "Little Sisters of Eluria" story (in order of how it appears in is life). After reading the comics, now I want to read the books again to continue following Roland's story.