There's an absolutely insane amount of content in this book. On top of the ridiculous number of spells that are worth the asking price on their own, there are the paths. The expert paths are all about new ways to interact with magic, with most of them being ways to combine two traditions. Some of them are obvious, with air/water being ice magic and fire/earth magic being lava magic, but some others are less obvious. Invocation/necromancy is a combination I wouldn't have thought of, but I'm looking forwards to putting together a Soulcatcher nonetheless.
Of course, there are options for the more mundane caster. Become a Magister to fight against dark magic practitioners, or focus on attack magic with the Spellslinger.
Each tradition of magic gets a new master path, as well. I've always found the Pyromancer lacking somewhat in the "wow" factor, but now I can play a Firebrand and light myself on goddamn fire. Perfect. Summon weapons made of pure magic with the Mageblade, use your voice to cast spells from afar with the Spellsinger, and see the future as a Watcher. No longer do you have to have your two earth magicians fight each other to the death in ritual combat over the Geomancer master path!
What about tradition-neutral master paths? You won't be left out. Don't feel like living an eternal life as an undead abomination? Show up those amateurs in their dumb black pyramids by becoming a Soulbound, gaining relief from the clutches of death without having to die first. Want to master your spells? The Orb Master manipulates area of effect spells like no other, the Spell Sniper is a master of attack rolls, and the Taskmaster devote themselves to utility spells.
If you're planning to run Shadow of the Demon Lord for any length of time, this is one of the first books I would recommend buying. I consider this added to the base game and the two Demon Lord's Companion books in what is essentially the "core" of the game.