I came to McGee late, and it was like a giant smack upside the head. He lays out, very clearly and cleanly, the assorted performance decisions you need to make when performing early music, and how you might go about making those decisions.
He covers reading fascimilies and modern editions; dealing with problems of tempo and meter (I didn't even know there were problems of tempo and meter); appropriate vocal style for solo and choral singers; instrument selection for instrumental pieces, ensembles, and accompaniment; creating arrangements for soloists, accompanied soloists and ensembles; issues of ornamentation and improvisation. Each section by itself is small and digestible.
Most importantly, this isn't all airy theory. McGee tests his ideas out in performance, and the work is very much a guide for performers, as the title indicates. He offers practical, solid guidelines to work from.
Two caveats. First, this is an older book (1990), so some of the performance practice suggestions may have been refined in the meanwhile.
Second, don't think of this book as either gospel or a straightjacket. It offers practical, solid guidelines, based on what appear to be the "norms" of medieval and Renaissance music. Any particular piece of music may be different from those norms. As Guido d'Arezzo (c. 1050) said in his treatise on music, "Do everything that we have said neither too rarely nor too unremittingly, but with taste." Respect the world the music came from (as we understand it from the scholarship) but also let each individual piece speak to you, and allow yourself to try things beyond these guidelines.
I would say that access to Hoppin's Medieval Music and McGee's "Performer's Guide" are an excellent and necessary start to any SCAdian's music library, whether the SCAdian in question is interested in solo (bardic), polyphony (choral groups), or ensemble (instrumental) performance. If I lost the rest of my library, I could do a lot with just those two.