I have never played the Ruy, as best as I can recall, but I have played against it many times. I enjoy playing against it. Plenty of people will disagree with me on my approach to learning any opening, but here it is:
First learn the basic ideas of the initial opening moves. Edward Lasker, in his book Chess Strategy, explains this very well. Then play it against a stronger opponent and then go over your game and figure out where you lost. Then next time figure out a play to overcome the reason you lost the last time.
In doing this I once played the Marshall over 15 moves deep having never studied it at all. I just realized I needed to play d5 earlier than I had and played it by ear. Actually I transposed into a Marshall from a Morphy (also which I had not studied but just felt like those moves should work).
The idea is not to memorize moves but understand what move you should play to answer your opponent's move. And getting beat by that opening and then studying why is a great way to figure out for yourself good defenses.
The one defense to the Ruy that I have studied is Bird's. And if you want to get somebody out of book real quick, that will almost certainly do it - 3...Nd4. A GM will tell you that it's an unsound defense, but I've beaten players rated 400 points higher than me using it. They knew a lot of moves in the Ruy, but they didn't know that.
But even with that, knowing WHY the moves are played frees you from having to memorize. If you know the why and figure out a good attack or defense using your own head (even if someone smarter had figured out the same thing long ago), then you don't have to memorize, you can always figure it out again.
The book by Horowitz is 40 years old and is in descriptive notation.
The exchange variation was used by Gata Kamsky to avoid the Marshall Attack, but Gata can use technique to squeeze a win better than many.
The Archangel Variation has been used in recent years to produce some interesting chances for Black initiative.
Reb's suggested 5.d4 is a way to avoid many variations.
My articles on this site covering the Ruy Lopez are (so far):
"Rusudan Goletiani" has 5.d4
"Ruy Lopez: Open Defense"
"Ruy Lopez: Open to Rudeness" has the Open Defense
"Ruy Lopez: Berlin Without the Wall"