What soy sauce is Tamari closest to?
Your typical Tamari is, to my understanding, going to be closer to dark than light soy sauce. So you'd want light as a second soy sauce. 

Inside Japan, there are five kinds of soy sauce:


Koikuchi, which is your most typical soy sauce
Usukuchi, which is lighter because it has fermented rice in it
Tamari, which has more soy than wheat and thus is a bit darker
Shiro, which has more wheat than soy and thus is a bit lighter
Saishikomi, which is "twice-brewed" and thus twice as dark


In China, you have two: regular "light" soy sauce, and "dark", which is the light stuff processed further and aged. Therefore, "sashikomi" is the Japanese equivalent to dark soy sauce, if you want to stick with Japanese classifications. 

If your recipes are aimed at english-speaking audiences, however, the terms are likely to be used a lot looser and less precisely than in Japan or China. Kikkoman, which is the biggest name brand I've seen in the US, brews a regular, a Tamari, and a gluten-free (Tamari can have some gluten in it). Therefore, I would normally assume that to an American chef, "dark soy sauce" is Tamari. "Light soy sauce" might mean the low-sodium stuff, or it might mean Shiro or Usukuchi. Your basic unqualified "soy sauce" is Koikuchi.