On the shortlist so far:
Interviewing Users: How to Uncover Compelling Insights - Steve Portigal
Talking to Humans - Giff Constable
Continuous Discovery Habits: Discover Products that Create Customer Value and Business Value - Teresa Torres
Interviewing for research - Andrew Travers
Practical Empathy - Indi Young
And yep I've already done The Mom Test by Rob Fitzpatrick:
Thanks for your suggestions!
Not as sexy as the other books but MORE Guerilla Marketing Research is a great book imho.
Ah cool, thanks for the recommendation Gökçe - hadn't seen this one before.
Oh also deploy empathy - Michele Hansen