First and foremost, this book offers two new core classes and a 3.5 update: the Warlock and the War Mage are both new, and the Wu Jen has been updated from Oriental adventures. Of particular interest are the two new classes and how they balance.
The Warlock doesn't have spells, but invocations, and these invocations are essentially at-will spell-like powers. So instead of the usual spell lists and spell slots, instead of spells, instead of spell points or psionic points, we have the first class completely built on at-will magical abilities. A Warlock at 20th level has only 12 invocations, total. Invocation powers vary from least (duplicating the darkness spell, for example) to dark (duplicating the foresight spell), and are group by four classifications rather than nine levels. (They do have spell level defined for purposes of saving throws and the like.) On top of the invocations, a Warlock comes with extra hit points (d6), gradually increasing damage resistance, and a built-in blasting power that does damage about on par with magic missile (at will, mind you) and can be modified into more powerful variations with invocations. I'm not sure yet whether it is well-balanced, but it appears to be so. I will likely use it as a villain at some point before I allow players access to it.
The War Mage is a rather intriguing design. The spell casting is like a sorcerer's with the same large amounts of spell "slots" per day, and an ability to choose what gets cast when, rather than having prepared spells. The class also gets to wear light armor without taking an arcane casting penalty, and this can be increase to even heavy armor at higher levels if one also spends a feat to improve this ability. Combined with this is a d6 hit die so this plus the armor makes for a tougher cookie than a wizard or sorcerer. The real interesting aspect of this class is in the spell list and how it is employed. The class automatically has access to every spell for each spell level it can access: spells known -is- the spell list. Further, these spells comprise the best combat spells in the game. On top of this, the War Mage gets an "Edge" that adds his intelligence bonus to the damage of any spell cast (which is about equivalent to adding an extra die of damage to every damage spell). So just by taking the class, you get to wear armor, have a decent d6 hit die, and can cast the best combat spells in the game. What a munchkin, right? Well, I haven't mentioned the trade-off, yet: it's -only- offensive combat spells. This class cannot cast extremely useful spells that are staples of the sorcerer/wizard spell lists. There is no Identify, there is no Detect Magic, there is no Dispel Magic, there is no Shield, there is no Mage Armor, there is no Fly, there is no Teleport, there is no Polymorph. You can blast things to kingdom come. That's it. The party had better have at least a bard to make up for some of these, but it would take a full-time wizard or sorcerer to truly make up for the lack of versatility. Ideally, I see the War Mage as a strong computer game class, because most of what is lost are the spells that just aren't useful in a computer game, because it is so difficult to write a game than can handle these aspects of a true pencil-and-paper wizard's versatility. I look forward to including the class in my gaming both as PC and NPC, because I find its strengths and limitations fascinating.
Several of the prestige classes from Tome and Blood are found here in new, revised versions, but there are several new prestige classes, as well. A couple of these are bard-oriented. Of particular note are the Argent Savant (a force-spell specialist), the Effigy Master (a special kind of golem specialist), and the Wild Mage (which hasn't appeared in official WotC material until now, as far as I can tell, and has only existed in independent d20 sources and in its old 2nd-edition version). The concept behind the Argent Savant is intriguing, and even though it is only 5 levels, if force spells like magic missile and shield are what intruigue you about wizard spells, it is very much worth taking. The Effigy Master is definitely a unique approach to magic, though a bit awkward if one doesn't enjoy writing up creatures from the monster manual and applying an "effigy" template to them. The Wild Mage is remarkable in a couple of ways, in that it was included at all, and that it has only a 2 page write up. Basically, the Wild Mage ends up with a variable spell caster level (average level is about .5 higher than an analogous non-wild caster) that can exert mild control over random spells and at high levels can take a memorized spell and turn it into a Wand of Wonder effect. I'm glad they took the time to consider adding the Wild Mage to the roster, but I think it deserves a lot more lovingkindness and attention to detail. Using randomized spell caster levels and the wand of wonder table is what allows the write-up to be only 3 pages. There is no chance of spells "going wild" by accident, which was part of the charm of the old Wild Mage, and there are no big tables of possible wild mishaps ordered from most disasterous to most helpful (so that a bonus applied to the role could be a kind of good luck nudge). I don't fault the designers for this, though. The Wild Mage as classically understood is not easy to balance. I think the Wild Mage concept is still undergoing growing pains, much like psionics did until the two most recent psionics handbooks, and we'll see some more official versions over the next several years until the kinks get ironed out.
Beyond the core and prestige classes are the feats, and there are several new feats that make this book worth having all by itself. Some of these pertain just to the new core classes, and others are simply latest revisions of 3.0 feats. The new feats are intriguing, however. Arcane Mastery lets you take 10 on a caster level check, meaning automatic success against casters of lower level (useful for dispels) and for always getting past weak magic resistance. Several feats offer triplets of 1/day spell-like powers (0 and 1st level). Mage Slayer is no longer a class, but a set of feats. Those doing the dragon thing have a bunch of Draconic feats they may take. You can get a familiar with a feat, if your core arcane spellcasting class doesn't have one. Practiced Spellcaster does a great service to all multiclass spellcasters: add up to +4 to your caster level (not increasing number of spells, but the power with which they are cast) so long as it doesn't exceed your character level. If you are a 4th level fighter, 6th level wizard, you can take this feat and cast your spells (still limited to the spell progression as of level 6) as if you were 10th level. Thus, you can do damage (and dispel magic, and penetrate spell resistance) based on your character level rather than on your much weaker class level. This does a world of good to balance the game and let players not be crippled for wanting to have a bit of variety in their characters.
As far as the spell lists in this book go, this alone is worth the price on the cover. As with any new set of spells, there are good and bad, balanced and imbalanced, but having new ideas for what spells can do added to the official material is always nice. There's even a cleric spell in here that I want to ask my DM with whom I am playing a cleric to allow.
Why not 5 stars? Simply because as with most game supplements (and even the core rulebooks), there is a lot of hit and miss. There's always things that I don't like, or think that could be done better, or I will have to house-rule for myself because the designers severely over- or under- powered something. Also, with so much variety, there are always elements that just don't apply to anything that one would consider playing; not to say that these elements are bad or wrong, just uninteresting or inapplicable to current plans. However, what I do like in this book, I -really- like, and there is plenty of that.