Chessable.com has a really nice intro book on the Ruy:
A simple book about Ruy Lopez
There's Mastering the Spanish if you can find a copy... it looks like there are used ones on Amazon. I wouldn't call it a simple book but it's broken up by pawn structures and deals with ideas more than concrete variations as you're asking.
Possibly helpful:
Starting Out: Ruy Lopez by John Shaw (2003)
https://web.archive.org/web/20140627024240/http://www.chesscafe.com/text/hansen53.pdf
The Ruy Lopez Explained by Gary Lane (2005)
https://web.archive.org/web/20140626201436/http://www.chesscafe.com/text/hansen80.pdf
The Ruy Lopez: Move by Move by Neil McDonald (2011)
https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf
"The Mammoth Book of Chess" by Burgess has a long section on the Ruy, aimed at our level. Plus it covers every other opening - useful if your opponent doesn't agree to Spanish torture...

There's Mastering the Spanish if you can find a copy... it looks like there are used ones on Amazon. I wouldn't call it a simple book but it's broken up by pawn structures and deals with ideas more than concrete variations as you're asking.
I agree with this. Daniel King's "Mastering the Spanish" is an excellent introduction to the Ruy Lopez. As in the other books in the "Mastering the ..." series, the chapters are broken down by the typical pawn structures and for each structure present the plans for both sides and the major tactical themes. Used copies are available on Amazon from $27.50. (Note that "mastering" in the title is a marketing gimmick: although the books are excellent, they just introduce you to the opening; they do not give you mastery of it.)
I'm not familiar with Neil McDonald's book, but it looks like it would meet your requirements, and McDonald is a very good author.
Books already are obsolete while being printed. The Ruy Lopez is a heavily played opening and right now thousands of games are being played with it and the evaluations are changeing. It is better to study recent annotated grandmaster games. For example at the recent Yekaterinburg Candidates' Tournament several Ruy Lopez games were played.

But wouldn't you rather have a massive book to play the Lopez correctly, since there are so many variations ? Is there even a point in looking at one peculiar line if you're not going to look them all ? If you're starting, it's fine to not look at theory right away in order to develop your own ideas and then either defer or confirm them, but you will need complete knowledge of the various tabiyas at some point.

Books already are obsolete while being printed. The Ruy Lopez is a heavily played opening and right now thousands of games are being played with it and the evaluations are changeing. It is better to study recent annotated grandmaster games. For example at the recent Yekaterinburg Candidates' Tournament several Ruy Lopez games were played.
You can't say opening books are obsolete, the theory evolves over time and that's something readers need to always be aware of and check with a database. Using opening books as supplementary learning material is fantastic
Dear All,
I am looking for a simple, introductory book on Ruy Lopez, something which is considerably more about common ideas and recurrent tactical motives rather than particular long lines to memorate. A substantial narrative (rather than moves notation only) whould be appreciated.
Any suggestions? Yours, Daimonion