You can buy good road maps in the fuel stations here, I have even seen "Photography maps" on sale lately. Though, it is all very straight forward, you could have a fullfilling trip just following the #1 road, but useful to know of a few hidden gems. Personally I prefer the North during Summer, but probably because I spend most of the Winter on the South coast.
I have a modest location guide on this site:Iceland Locations
Most of all stay safe and have fun... then come back in the Winter.