I've read a few books by Les Daniels, now several years ago. Daniels wrote the books that I still refer to for historical context on DC's Trinity (I pulled the Superman book off the shelf just last month for some fact checking). I've always understood that he preceded current historians like Hajdu or Gerard Jones, and its certainly not the place where one earns glory in comics.
Likely Daniels' greatest exposure came in the creation of the DC Masterpiece box sets that came out a few years ago that you could pick up at Barnes & Noble or Borders. Those things were actually pretty amazing, and if you're a comics fan, you missed out if you didn't grab them.
Mostly, I appreciated Daniels' approach as an historian, not really shying away from some of the goofier or odder sides of the development of characters. It was from him that I first read about Wonder Woman's origins derived from William Moulton Marston and Marston's particular proclivities. Its not an approach 95% of the folks currently writing about comics seem to get (or want to get), that these characters we love do not spring whole cloth from the imagination, but from the forces of the time, the forces of personality of creators and the environment and culture that formed the minds of those creators.
In the end, I don't have much to add about Daniels, other than that he opened my eyes to the sweeping history within the publication of superheroes, and I suspect that he helped build my fascination with Superman, Batman and Wonder Woman as more than characters on a page, but as icons of western culture in their own right.