What you describe is the best advice to meeting an unusual opening: play by the principles.
In terms of resources, there are 2 I can think of:
- Beating Unusual Chess Openings by IM Richard Palliser. By 'unusual', Palliser really means anything other than 1. e4 or 1. d4, so he covers from Black's point of view:
The respectable (English Opening, Reti and King s Indian Attack) through to the offbeat (Nimzo-Larsen Attack, Bird s Opening) and the totally bizarre (Orang-utan, Grob)
- Chess.SE. We are always happy to see questions such as What are the ideas behind the Blackmar-Diemer gambit?What are the ideas behind the Blackmar-Diemer gambit?