I've got a lot of opening books, but none like this.
Marin's Beating the Open Games, and A Spanish Opening Repertoire for black combine the best features of an informant encyclopedia style layout, and English descriptions of the concepts.
Each variation is summarized at the end of every chapter in an Informant Encyclopedia type layout. The notes on the moves are in English and offer far more explanation than someone would get from an Informant Encyclopedia. In addition, the main concepts are shown using games and verbal explanations prior to the Encyclopedic layout at the end of the chapter.
Amazingly, no one else has ever organized an opening book this way. It's always been one or the other. Games with notes, or Informant like dumps with symbols.
Both books are essentially repertoire books for Black, but players on the White side will get some benefit as well. I'm sure a lot of these lines will start showing up in tournaments in the future.
For the main line of the Ruy Lopez (Spanish Opening), Marin offers two systems for Black, The Rubenstein system or the Petrosion system, so don't expect to learn the Marshall attack. Even if you have no desire to play the main-lines that Marin offers, this book has the best analysis available for a lot of sidelines White can throw at you.
I actually switched my repertoire from the Sicilian (1.e4 c5) to 1.e4 e5 temporarily because of Marin's two books. They offered a simple and effective way for me to initially study that line of play without having to refer to a lot of other sources. I wasn't completely happy with using his Rubenstein recommendations however. I'll probably try the Petrosian system next, or perhaps find something that covers the Marshall attack to complement Marin's analysis.
I believe these books are probably effective for anyone rated from 1400 on up to even Grandmaster. The concise Informant like tables that document the lines at the end of each chapter will appeal to stronger players and the early chapter explanations with sample games are good for anyone learning the opening.