I'm looking for good maths textbooks (preferably with olympiad style problems as the harder exercises) which cover uncommon techniques in maths. An good example of the type of book I'm looking for would be Yaglom et al's book on induction in geometry.
I hope the question isn't too broad, thanks for your help.
This book Mathematical Problems and Proofs: Combinatorics, Number Theory, and Geometry is very good with many examples and exercises.