I recently finished reading a new book on the National Parks, Complete National Parks of the United States by Mel White, published by National Geographic. I have quite a library of National Park books, but this one was a fine addition. When they say complete they mean complete. Not only does it cover the National Parks, but the National Monuments, National Seashores, National Historic Sites, and so on (25 different classifications).
The book covers the history, purpose, features, and things to do and see at each park. The entries are well-written and detailed enough so you know what the park is like and whether you would want to go. Longer more detailed entries would not be a good idea, as the book is already 528 pages and printed on heavy paper. There are maps for all the National Parks, but unfortunately not for the other units. There are color photos throughout.
There are many books covering the National Parks in every possible level of detail, but finding a book about the other units in the system is not so easy. This book has it all and is well-written and well-designed. If you want one book on the National Parks, this would be the one. The list price is $40 (hardcover), but I got mine at Amazon for $26.40.