Natural causes of language by Nick Enfield discusses theories behind cultural transmission of language. From the blurb:
What causes a language to be the way it is? Some features are universal, some are inherited, others are borrowed, and yet others are internally innovated. But no matter where a bit of language is from, it will only exist if it has been diffused and kept in circulation through social interaction in the history of a community. This book makes the case that a proper understanding of the ontology of language systems has to be grounded in the causal mechanisms by which linguistic items are socially transmitted, in communicative contexts.
I like the argument that a particular 'language' (like English or Welsh) is not a real entity, but a "convenient fiction" - something I also argued in my thesis.
It's a special book in two senses. First, it comes from the new Language Sciences Press: an open access publisher where publishing costs nothing to the author and reading costs nothing to the reader. Hopefully we'll see this being used to good effect.
Secondly, it comes with a video introduction from the author!