I stumbled upon a copy of 'CodeNotes for XML' and I just love it. It's a smallish book (like a pocket guide feel) with only a few hundred pages that explains XML just beautifully and offers a great layout for a handy reference book.
The book was great at filtering out fluff that you find in most books but yet it did the job just as well/better when it comes to actually learning something.
By the looks the series only lasted during the early 2000's but I would really really love to find a similar series (or know if they just changed the name of the series).
You can view the books HERE