You might want to check out the great apocrypha collection; it contains the following on spell guidelines:
HoH:S - New spell guidelines and rules are provided. Rules for firing projectiles using Rego are given on p. 35 and p. 38. Guidelines for rego and creo craft magic are given on p. 60. Rules for flourishes (flourishing your sigil) are given on p. 60-61. Guidelines for illusion and mental magic are given on p. 61-70. A suggestion on how to treat Aura of Ennobled Presence and similar spells in combination with the Gift is given on p. 96. A new healing guidelines is given on p. 100. A new ReVi guideline is given on p. 129.
Covenants - Spell guidelines regarding rego craft magic are given on p. 49-50. Further guidelines involving various topics (used in "agriculture" spells) are given on p. 50-51.
HoHTL - New spell guidelines are provided. Ones related to investigation magic are on page 72-75.
TMRE - New spell guidelines are the numerical guidelines (open to all) on p. 93. There are also the spirit summoning/control guidelines, on p. 28.
RoP:I - Demonology related spell guidelines are on p. 121.
There are a few other entries that aren't quite spell guidelines but you might find interesting (such as guidelines on treating pregnancy and so on).