It depends at the level, I think for 1500-2000 these books will be usefull.
Chess Strategy for Club Players by Grooten
Chess Secrets: The Giants of Strategy by Mc Donald
Beat The Grandmasters by Kongsted (a tactic book)
Understanding Chess Endgame by Nunn
Secret of Chess Endgame Strategy by Hansen
Understanding Pawn Play In Chess by Marovic
1001 Deadly Checkmates by Nunn
Survival Guide To Rook Endgames by Emms
hi i am looking to beef my chess book collection up a bit and wondered which are deemed essential and likely to boost your rating?