One of the biggest changes between the first and second editions of Warhammer Fantasy Roleplay was the magic system. Driven mostly by changes in the background of the miniatures wargame in between the two editions, the second edition ditched first edition's system of leveled spells and magic points for the new system of color magic. Most of the basics are covered in the main rulebook, of course, but Realms of Sorcery fleshes that out in nearly every direction.
I'll come back to that "nearly."
As you might expect, there's a lot of fluff in the first half of the book. There's a history of magic usage in the Empire up to the point of the Great War Against Chaos, which I can easily summarize as "BURN THE WITCH! Priest are okay though." During the war, Emperor Magnus the Pious sent for aid to Ulthuan, and the High Elves sent three archmages, which was all they could spare at the time. Those archmages went on to help win the war by teaching humans how to safely become wizards, but unlike elves, humans can only use one color at a time safely. Hence the eight orders.
There's a bunch of exposition about the orders as well, with some neat tidbits. Members of the Amber Order don't have a college in Altdorf like the others, instead lairing in the hills outside the city walls. Members of the Grey Order take strong vows never to use their magic for venal financial gain, precisely because it would be so easy for them to do so. The colleges themselves are well-described, too, in a way that lessens their impact on the landscape of Altdorf. I've read that a lot of people don't really like the Colleges of Magic, because they feel like their overtly high fantasy feel damages the presentation of the Warhammer world. I can see that, but there are some colleges that I think actually make things even more mysterious. Like the Azure College, which is a huge building with plenty of high towers to see the stars, but which is never actually visible due to the workings of fate--anyone who looks at it will get bumped into, or trip, or laundry will blow in front of it, or the person will think of something else they have to do, and so on. The Bright College is in the middle of a burned-out stretch of ruins that Altdorfers refuse to move back into, and the Amethyst College appears as a building that's been deserted for decades unless you actually have legitimate business there, in which case you'll probably turn a corner and meet a magister. Or the aforementioned Amber College, in a series of caves. I think it gives the proper mysterious touch to magic that first edition didn't really have.
Then there are the mechanics sections, which I think are really valuable. One of the problems with the spell list system for color magic as of the main book is that because a wizard gets all the spells they would ever learn immediately on taking the Arcane Magic Talent, the whole idea of knowledge-seeking wizards pouring through ancient tomes of arcane lore is restricted to rituals, and the example rituals given in the corebook leave basically no reason why anyone would actually want to cast them based on how difficult they are to use. Honestly, it's probably easier for a Bright Wizard to just set a town on fire than to gather all the materials to use The Awakening of the Slumbering Earth Dragon. The addition of ten extra spells, a choice of multiple lists (each of which only has ten spells), and the Extra Spell Talent to learn the other spells provides both an XP sink for wizards and a reason to seek out knowledge.
There's also a section on witches and witch-hunters, which is short but does a good job.
Finally, there are parts about alchemy, wizards' familiars, and magical items. The alchemy chapter has a very Warhammery (if I can use that word) take on alchemy; potions, being made of perishable ingredients, have a shelf life and can go bad in all sorts of hilarious ways. Familiars provide bonuses for the wizards who use them, but there's a great table of personality descriptions of the familiars to provide some character to them, including options like "Passive-Aggressive," "Know-It-All," and "Raving Mad." There's options for constructed familiars as well as natural animals, so creepy wizards can have their homunculi. The magic items is mostly just a list--in keeping with their rarity, there's no standard rules for making them--but it's nice to have options.
Now, the problems. One of the major problems I had with Realms of Sorcery is its breadth. It's pretty much entirely focused on Imperial magic, and not only that, on modern Imperial magic. I find it really bizarre that there were never any successful wizards in the 2300 years prior to Magnus the Pious, and kind of sad that the other traditions from first edition, like druids or elementalists, weren't included. It does make a nod to druids in the backstory of the Jade Order, and I suppose that the various colors of magic replicate the feel of elementalism--Bright is fire, Azure is Air, Jade is Earth--but it does hammer down the type of acceptable characters to a very defined set. Especially since Tilea, Bretonnia, Estalia, and Kislev exist and presumably have their own type of wizards, but they aren't defined. Kislev does get a breakdown of its magic in Realm of the Ice Queen, but none of the others ever did. It's a persistent problem with the WFRP stuff being so Empire-centric.
The other problem is elves. The book implies that elves should have mechanical differences in the way they interact with magic, but there's no hint on how to handle that. Despite elves being able to use multiple colors without the apparent certainty of harm (or at least, of going crazy and turning evil) that humans have, they apparently still only have the same Apprentice Wizard career that humans do. Unlike Tileans and Estalians, elf wizards had a direct and obvious effect on the magical development of the Empire, and the complete lack of mechanical support for that was pretty disappointing to me.
Other than those points, it's a great sourcebook, and I think it'd be highly valuable for background and antagonist info even in a game with no PC wizards.