Which passport number should I use?

Question

I have 2 passports (one expired with a valid visa and one other passport valid); while booking a ticket, which passport number should I put in?

Answer 1

You use the passport number of the valid unexpired passport. The passport is the premier travel document. The expired passport is carried along just to provide evidence of the previously issued valid visa

Reference

Extensive personal experience with similar scenario while visiting different countries.