My favorite book on customer development is "The Mom Test"[0], by a YCombinator alum. The name is terrible, but the content of that book is golden. He tells the terrifying story of talking to customers constantly as they wasted a million dollars building something the customers were not going to buy. There are various ways the customer pays you before the product is ready, and if they aren't paying you, they aren't really interested. Payment can be in the form of time (they meet with you), personal reputation (they recommend you to friends), company reputation (they arrange a meeting with their boss, or coworkers), etc.

[0] https://www.amazon.com/Mom-Test-customers-business-everyone-...