Bibs and IsaVulpes are right on the money. 

As I mentioned in the other thread, Dangerous Weapons: The Ruy Lopez also has coverage of various d3 lines. There's a chapter each on 3...Nf6 4.d3, 3...a6 4.Ba4 Nf6 5.0-0 Be7 6.d3, and a surprisingly sharp way to play 3...a6 4.Ba4 Nf6 5.d3 b5 6.Bb3 Be7 for White.

In the first chapter of McDonald's Move by Move book, he sketches a d3 based repertoire with 5.d3, 3...f5 4.d3 and against 3...Nf6 both 4.d3 and 4.Qe2 followed by c3 and d3. A useful start in the Ruy Lopez, but this is definitely on "Starting Out" level. (The rest of the book covers main lines with d4 as well as Black's various sidelines).

The Greet book Play the Ruy Lopez recommends 5.Qe2, the Worrall Attack, to avoid the main lines. That's not a bad addition if you want an alternative to d3 but are still allergic to the main lines for some reason. 

Not a book, but Svidler has a video series from the White side on 6.d3 on chess24. It is wholly honest (not pretending to gain an advantage in each and every line), covers Black tries that at the time of creation weren't common at all (and eg Ntirlis made one of those suggestions for Black his own recommendation in his 1.e4 e5 book), and is rather cheap for what amounts to being opening files by a 2750 on one of his main weapons.
https://chess24.com/en/learn/advanced/video/the-6-d3-spanish-according-to-svidle...

He covers the a3-plans, which Anand originally popularized and which are seen as the most testing currently; so if you wanted a d3-c3 system, you'd need to look elsewhere. 

It does however not cover Black deviations prior to the position after 6.d3, so the Open Spanish, Steinitz, Nge7-Systems etc, you'd need your own stuff against / a different source for.