A deliberately gentle, non-technical introduction to the topic. Every item in the small and carefully-chosen bibliography is worth following up.
Hunt & Black: Unit selection in a concatenative speech synthesis system using a large speech database
The classic description of unit selection, described as a search through a network.
Bennett: Large Scale Evaluation of Corpus-based Synthesisers
An analysis of the first Blizzard Challenge, which is an evaluation of speech synthesisers using a common database.
This is the new version. Still under construction.