My favorite is the classic "Positioning" by Ries & Trout, a must-read.

I wish I could recommend others but there aren't a lot of great strategic marketing books out there in my opinion.  To me real-world case studies are more useful - "Behind the Arches" the story of McDonalds is a great one, although not travel related.

Let me know if you come across anything you'd recommend

Thanks,

Ted