1) H.D. Gardeil's Introduction to the Philosophy of St. Thomas Aquinas (vol. 4 is on Metaphysics; and unfortunately it is also out of print), and
There are also many recent scholarly works on this doctrine of participation, but on the whole they tend to be a hardheaded interpretation of St. Thomas from the point of view of existentialism and/or Platonic metaphysics. Examples of authors that read Aquinas this way are Cornelio Fabro, Joseph De Finance, and Norris Clarke. This is generally called the Existentialist Thomist School.
By the way, if you are up to it, there is an awesome, 4-volume Latin work on Analogy by one of the greatest traditional Spanish Thomists: Santiago Ramirez, De Analogia. It is very difficult to find through American libraries--your best bet would be to order it directly from the Editorial San Esteban (Spain).
I hope this helps.
Francisco J. Romero
Professor of Philosophy, Theology and Languages
Our Lady of the Sacred Heart Academy
3218 11th Street
Rockford, IL 61109
Alt. Email: Francisco.Romero@marquette.edu