Of purely historical interest, this is an example of a system using a heterogeneous unit type inventory, developed shortly before Hunt & Black published their influential paper.
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.
Clark et al: Multisyn: Open-domain unit selection for the Festival speech synthesis system
A description of the implementation and evaluation of Festival’s unit selection engine, called Multisyn.
Taylor – Chapter 16 – Unit-selection synthesis
A substantial chapter covering target cost, join cost and search.
This is the new version. Still under construction.