I've never written a review of an RPG product I've purchsed before, but fact that this book is so all over the place has compelled me to act.
Book of Secrets opens up with a re-print of some character Archetypes that existed in older editions. I find the update to be useful as I do not have easy access to a lot of older edition material. In the same chapter, the book adds a ton of interesting merits and flaws so you can get your character just right, though the actual quality of these merits and flaws varies.
Still in this same chapter, the book gives a whole bunch of rules for both martial arts manuevers and hacking computer systems. These are not necessarily improvements to the game, but a story focused on one or the other will certainly benefit from them. I know hearing "Roll Intelligence + Computers" for the 100th time in a game that revolves around busting into that Iteration X construct may get a bit boring.
The section on Wonders is great, giving a lot of useful information on different types of wonders (and how people coming from different paradigms view wonders). An optional, more complicated system for making wonders is introduced that can be extremely useful for making the creation of a wonder a central plot of a game rather than something done in off-time. A number of sample wonders are given to provide examples (somethings I always appreciate).
Following this, there is a chapter that greatly expands focuses, including new paradigms, practices, and instruments. The book even gives sample characters (with pretty nifty full color art) that show how these things combine into a focus.
Finally, there is a setting chapter that describes how the different major magical alliances (and even the nephandi and marauders) handle justice with their members. Given the propensity of players to end up going against their traditions/conventions/crafts, I found this section useful as it takes the burden off me if I am wearing my GM hat to come up with something. Participating in the justice system of hidden wizard conspiracies can also be an adventure in its own right.
Mage is famous for getting a bit bloated with rules, and unfortunately Book of Secrets does nothing to buck this trend. Space in RPG splatbooks is limited, so spending 16 pages on a list of secondary skills that no one will ever put a dot in is perhaps not their best use of that space. It's not even the most egregious use of this waste (see The Ugly below for that travesty).
There is a whole section on expanding rules for resonances. Unlike useless skills (a pet peeve of mine), resonance is something where I am willing to admit that others' mileage may vary. However, in my experience the mechanical system behind resonances is a very tedious aspect of the game that pretty much always end up slowing down play rather than adding something to the setting. BoS adds a whole bunch of new systems on top of the one mentioned in M20, including a whole other category of resonances (called Synergy), so if you want to use all published rules you had better fire up that spread sheet program. Not really my cup of tea; if your character is creepy, or joyful, or whatever, we don't need a system to track exactly how creepy or joyful he is, you can just describe it in your actions and magical effects.
Chapter 5 is why I am writing this review.
I can't believe I am actually typing this out, but this chapter (43 pages) is dedicated to answering questions and criticisms of M20 from the online community (aka, forum posters). It ranges from answering the boringly technical that can easily be adjudicated by a GM ("Can a mage cast multiple effects in a single casting?"), to the borderline insulting and ignorant ("Why does M20 feature so much liberal politics" - the insulting and ignorant parts here are in the answer, not the question), to the the head-scratchingly baffling ("Why Didn’t You Detail the “Camps” – That is, Groups Within Each Tradition, Technocratic Methodologies, and So On?" - I guess because instead of using 43 pages on this in a book where it would have been perfect, we have to respond to people who are wrong on the internet).
This whole farce starts with addressing a compaint that Mage is stuck in the 90's, which when I was done reading I could only think about the author "You can take the malcontent Gen-Xer out of the 90's, but you can't take the 90's out of the malcontent Gen-Xer."
There is a section of chapter 5 where there is some advice on running M20 in different RPG/literary genres, as well as a list of inspirational material, that is ok, but boy is this chapter a doozy (and a waste of paper).