"...if I have both AE and Transcendence, does Mystic Secrets bring anything additional to the table? I couldn't really tell, as its blurb seemed a bit general, and I thought most of it would already be included in AE."
Nothing from Mystic Secrets was reprinted in AE. Basically, it's a supplement that expands upon the idea that Ceremonies/Ritual and Runes are fundamental parts of the setting by giving DMs and players more fluff and crunch beyond what AE suggests. It includes:
* Rituals/Ceremonies that any character, even a 1st level commoner could perform if they had the money and the time, granting small but often long-lasting benefits.
*Bonding Ceremonies that allow you to forge mystic ties to your chosen weapon or to your friends, giving you unique powers.
* New runes for Runethanes & Runelords.
*Minor Rune Manifests which can be summoned by anyone knowing the proper ritual. Halfway between Elementals and Items, these mystic forces grant unique powers to those who summon them...for a small fee.
* 4 new variant paths for the Runechild, focusing on Magic, Stealth, and Martial Prowess, as well as feats to help build upon their powers.
* Heralds of Annihilation, the equal and opposite of Runechildren. Great ideas for Campaign Villains that grow with the PCs from level 1-20
* Magical Locales. Includes sample locations within the Diamond Throne, as well as unique location traits that the DM can use to spice up his own creation. (WotC did something similar in DMG II)
* New Spells - a couple dozen new spells, from Knot to Earth Hammer and Tidal Wave.
* New Runic Equipment- Rather than standard enhancement bonues, these runes can be applied for a flat cost to any magical item, not just weapons and armor, but wands, rings, or clothes, what-have-you, to grant unique and powerful abilities.
Basically, it's an Options and Equipment book, with a heavy emphasis on mystical advantages you can get for skills and money in the Diamond Throne. Overall, I've found that it's a great aid if you're using the core Diamond Throne setting, but that it's mileage varies greatly in homebrew settings.
Robert "Yeah, I'm not Odhanon, but I figured I'd contribute anyway" Ranting