I love the book, and the concepts outlined in it. I've run into a few glitches, math errors, and oddities, that I plan to houserule, however.
Preternatural tethers don't seem to work properly. You take the highest, then add one for every tether that's at least half as strong. So... imagine this situation:
A level 2 preternatural tether, and three level 1s. That would be Aura 5, then.
Now, raise that level 2 to level 3. Suddenly, the three level 1s no longer count. So... by making something more magical, you actually cause the aura to DROP 2 levels.
That doesn't seem right.
Magic Might seems too disadvantageous - or rather, it seems too big an advantage to stay at low or zero Might, especially for a Magus with 'Transformed (Being)'. At zero Might, you can live indefinitely, ignore warping, get extra seasons out of not having to sleep, and pick up some qualities for fun. In return, you have to eat one pawn of vis a year, or live in an aura. You get a -0 non-penalty to experience gain. And it's far, FAR cheaper to gain Transformation experience points from vis than it would be if you gained Might.
Also! I caught the jokes in the 'nailing chamberpots' thing with the Vim magical humans, and the 'empedocles was sometimes right' thing about Criamon and Twilight. Were there any other little in-jokes and easter eggs that I missed?
A couple other things. Several powers, qualities, or virtues are listed that don't seem to exist. For example, Atlanteans have something called 'Surface Form' (which appears to change them from fish to merfolk when they're above water?) and several beings have things like 'Master of Form', 'Ruler of Form', etc. I assume those latter are Focus Powers, by analogy with 'Crafter of Form' which is defined, but it doesn't say.