opening question

https://www.chess.com/article/view/should-we-trust-computers

"... In the middlegame and especially the endgame you can get a long way through relying on general principles and the calculation of variations; in the opening you can go very wrong very quickly if you don't know what ideas have worked and what haven't in the past. It has taken hundreds of years of trial and error by great minds like Alekhine and, in our day, Kasparov to reach our current knowledge of the openings. ..." - GM Neil McDonald (2001)

I would add that, at one time, Steinitz disapproved of the Ruy Lopez, thinking, of course, that Black was fine when using the Steinitz defense. Years of experience demonstrated the strength of 3 Bb5.

For more on 3 Bb5, one could look at The Ruy Lopez: Move by Move by Neil McDonald (2011).
https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf
3 d4 is discussed in Playing 1.e4 - Caro-Kann, 1...e5 and Minor Lines by John Shaw.
http://www.qualitychess.co.uk/ebooks/Playing1e4CaroKannandothers-excerpt.pdf

Perhaps it would be better to get an introduction to both and other opening ideas by looking in Discovering Chess Openings by GM John Emms (2006).
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf