As Thomas Morley's "Plaine and Easie Introduction to Practicall Musick" is referenced in the "new polyphony" article in the latest issue of Sacred Music, I thought readers might enjoy this link to a PDF of the 1771 reprint of the book. Morley's book is particularly helpful in understanding prolation and proportion in English music of the late Renaissance, as are his "rules of composition," presented in the latter parts.

IMSLP is a wonderful resource for public domain music, but perhaps this has already been mentioned in other threads.

http://imslp.info/files/imglnks/usimg/6/6f/IMSLP03933-Morley.pdf