The Spanish opening or the four knights variation

Probably the Four Knights Spanish variation.

Either one is worthwhile. I'm currently studying the Four Knights as an alternative to the Vienna or Giuoco Piano, and there are a lot of similarities so that if you later want to look into those two openings, you'll find some familiar positions.

Reading reviews, Jan Pinski's The Four Knights c.2003 seems like the best single book.  I love that it has three different 13- to 18-page chapters on the Glek System (1 e4 e5  2 Nf3 Nc6 3 Nc3 Nf6 4 g3 and usually 5 Bg2) which looks tame but has some explosive features that aren't familiar to many.  For more on the Glek 4 Knights see:

http://chess-brabo.blogspot.be/2015/05/the-glek.html

and

http://kenilworthian.blogspot.com/2014/03/glek-four-knights-and-paulsen-vienna.html

I would be comfortable with the Spanish as white, it will probably transpose into the four knights anyways. It is a solid and simpler opening.

Don't forget when you're black, have a response against the Italian( Bc5) and the Spanish(a6 or d6) and study the queens gambit, since that shows up a lot. 

Possibly helpful:
The Ruy Lopez: Move by Move by Neil McDonald (2011)
https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf

For more on the Four Knights, one can try The Four Knights: Move by Move by Cyrus Lakdawala (2012)

https://web.archive.org/web/20140627104938/http://www.chesscafe.com/text/hansen159.pdf

https://web.archive.org/web/20140627040728/http://www.chesscafe.com/text/ebcafe06.pdf

and/or The Four Knights Game by Andrey Obodchuk (2011).
https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf