I'm disappointed with the Tome of Secrets. While the art and layout is fine, the descriptive content is pedestrian and terminally generic.
An example from the Warlock background description demonstrates the uninspiring prose throughout the book: "It is not known how or why the magic picks them; it just does."
Design suffers as well. As an example, the Warlord is capable of improving the fighting capabilities of fellow party members within a specified distance. That is all well and good. However, the designers then unnecessarily complicate the class by basing area of effects on different formulas:
Combat Leader: 10 feet per Charisma modifier + 1/2 class level.
Commanding Presence: Within seeing distance.
Inspiration: 30 feet.
Rallying Presence and Onward to Victory: 10 feet per Charisma modifier.
There is so much clean up work needed here that I can't practically used Tome of Secrets in my game.