The Book of Shadows is the players guide for Mage 1st edition and beyond. The book is broken down into some major sections and I'll look at them in turn:
Part 1 - Secondary abilities and merits/flaws. This section seemed great at the time but I feel it's hard to justify in modern play. M20 took the stance that secondary abilities are more or less specialties which should be cheaper to acquire and mostly a flavor aspect which should rarely come up. In the Book of Shadows, there's some ambiguity as it's hard to say "no, I don't want additional dots in dexterity or alertness but would rather put dots into quick draw". The logical response is "well, that's min maxing and you shouldn't do that in Mage". If a group weren't aware of these abilities and the scenario came up, the group would substitute another ability in and the game would proceed. Strangely, adding these secondary abilities makes the characters less powerful as there are more domains to spread ones dots over. Eh.
Merits and flaws were introduced in Vampire to kind of help tell characters apart but Mage has paradigm in addition to splat/faction plus ones mortal life so there's less of a need to have merits and flaws to tell one Chorister from another. Two Brujah may be anarchists, but two Choristers even both be Christian and have a world of difference due to denomination and such. Again, many of these merits kind of pay for themselves promoting abuse. Quick learner pays for itself in three stories, and sphere adept counter-intuitively encourages a player to pick few dots in their adept sphere so they can save on them in play. Some of the merit and flaws also don't work super well with Mage as a concept. Blind and Deaf feel they're easily overcome with one dot sphere effects. Again, a lot of opportunities for min maxing. While I recognize that's discouraged, it's possible for the system to help avoid this problem by making it harder.
One thing to think about, though, is the use of merits and flaws to flesh out NPCs. Things like Truth Faith are largely flavor for a player but could be vital for an NPC.
Part 2 - Outlines short fiction pieces focusing on each Tradition and faction outside the Traditions. These are largely fine. Each is three pages and includes an illustration which is also largely fine. These pieces tend to hew to stereotype so don't really add much depth here.
Part 3 - The Ahl-i-Batini. This section is fascinating and provides an interesting peek into a group where mortal complications led to difficulties in a Tradition. The one annoying fact is that it makes mention to the Night of Fana occurring in the 500s BCE and involving dervishes. Dervishes are a branch of Sufi Islam which would come about for 11 centuries. Eh, the 90s. The Ahl-i-Batini are a secretive sect and their methods are outlines beautifully.
Part 4 - This section outlines some systems and, while at the time was useful, there's little here that moves forward into later editions. In M20 almost everything is integrated into the core book and this is only necessary if you're playing something prior to about Revised which generally updated the rules. The details on the umbra are shallow and unnecessary if you have access to later books.
Detailed rules are then offered for familiars, Do, and Certamen. These are kind of niche cases but neat but Do comes across as overpowered.
This section is then rounded out by a discussion of rotes and talismans. These are useful and the sphere usage largely holds up to future scrutiny. Again, most of these are folded into M20 and are largely moved into that world whole cloth.
Part 5 - Parables. These are lovely little pieces of allegory about the mage's path to ascension. They're quite cute although not directly usable. I have used these in a game as a direct player reading and for that, it's pleasing.
Later in this section are short fiction pieces focused on the various aspects of the game like paradox, quintessence and tass, and the avatar. These are hit or miss. The paradox one is quite neat, but the others are meh.
Part 6 - Essays by various contributors. These add very little and I don't feel the monomyth holds up.
Overall, I have trouble recommending this to anyone who plans to use M20s copious rule set. Prior, it's kind of a necessity unless you want to roll your own for those.
Additionally, the PDF version is terrible quality and many of the sections with dark backgrounds are unreadable.