What books or approaches have you found to be the best for validating your market and/or conducting market research?
I think "The Mom Test" is the best market research approach and book I have ever seen. The goal should always be to validate your market before development. The realization that most people will mislead you to protect your ego when you speak to them about an idea you have is a revelation when it comes to market research.
I would highly recommend reading or listening to "The Mom Test" before you start development on your next idea or before you undertake any sort of development.