Lean UX by O'Reilly Media - This is a good read for a better understanding of the lean UX principles but seems to focus on teams with a few more resources than a startup. However, many of the principles can work within a startup if adjusted to account for it.

http://www.amazon.com/Lean-UX-Applying-Principles-Experience...

Don't Make Me Think by Steve Krug - This is one of my favcorites. Steve challenges you to really think about how you are building a product (mostly web) so that your users have the most frictionless experience possible and ensureing that things are made obvious to them.

http://www.amazon.com/Dont-Make-Think-Revisited-Usability/dp...

Why We Buy by Paco Underhill - This book isn't so much about web or digital focused UX as it is for physical space. Specifically, retail. It gives you great insight into the thought processes behind consumers and how some stores take this into account when designing their own store layouts. Very interesting read for any UX Designer.

http://www.amazon.com/Why-We-Buy-Shopping--Updated-Internet/...

Storytelling for User Experience by Whitney Quesenbery & Kevin Brooks - This one I grabbed tidbits of useful info out of here and there. Gives you a better understanding on just how to better build user stories for your product as well as usability testing etc.

http://www.amazon.com/Storytelling-User-Experience-Whitney-Q...