What would be the best four or five books to buy to learn programming using Squirrel specifically as relates to Electric Imp.
Thanks.
I do not know any books specifically about learning Squirrel or Electric Imp programming 
I would work through the Getting Started Guide, Squirrel Reference, and the Developer Guides.
Why did squirrel got chosen to be the imp language?
Why did it beat the competition?
Javascript is regaining a lot of popularity because of nodejs. Is squirrel better?
Squirrel is object oriented, event driven, and has a very small VM - all important aspects of the Electric Imp platform.
Javascript is not (really) object oriented, and it has a signficantly larger VM than squirrel. Larger VM’s require more overhead, which means less space for user code, etc.
We looked at some other languages, but ultimately Squirrel was the best choice for the feature set we were looking for (and it is similar enough to c and javascript so as to not be a significant hurdle for most developers).
Learning Squirrel is relatively easy - learning everything you can do with the Electric Imp API… that takes a bit more effort, and that would be true regardless of the language we selected simply because of the amount of functionality we expose.