I am venturing on the same journey. After 25 years playing chess, last year I decided to learn the Ruy. Too late you might say? No! I plan to be playing chess for at least another 25 years, so I might have learnt it by then

My starting point was Emms' "Easy Guide to the Ruy Lopez". This is a misleading title since it offers an uncompromising Mainline repetoire against all defences, offering a choice against some lines. Written in 1999, the lines still stand up well, however some latest Black tries e.g. 9. ...Bd7 in the Berlin defence endgame aren't there.
I've also got Khalifman's "Opening for White According to Anand" as a reference source since it is so detailed and fills in the missing lines in Emms book.
The best book on the Ruy IMO is "Mastering the Spanish with the Read and Play Method" by King and Ponzetto. It's a pure themes/strategies/ideas book. I wish more books like this were around.
I also have Gary Lane's book, which is next on my readng list.
As for the Petroff, I've used 5. c4 as suggested in Kaufman's "Chess Advantage in Black and White". An IM friend of mine suggested 5. Nc3 as an easy way to play for White if you don't want to learn lots of Theory. If you want to go main line though, there is the Khalifman books plus Kotronias's beating the Petroff, the latter being good in ideas/strategy.
Good luck on your journey. You are following the path of many great chess players, and many are travelling with you now.