I'm looking for some information on the ideas or themes behind different opening lines. E.g. Queens Gambit / Slav defence .. white tries to accomplish X and black tries to do Y without books of theory covering every variation. I'm just looking for general ideas e.g. White's black bishop is very strong - Black will try to trade it off ... or fight will be around isolated pawn etc etc. thanks (I'm a 1700 player)
I highly recommend the book Fundamental Chess Openings by Paul van der Sterren. It is a comprehensive survey of the chess openings with an emphasis on themes.
Understanding the Chess Openings by Sam Collins is a similar book and is also pretty good but FCO is more comprehensive.
The original book along these lines was The Ideas Behind the Chess Openings by Reuben Fine but it is very dated now (it was originally written in in 1943).