ToME: the Tales of Maj'Eyal

I would have thought I could stack speed, from Blinding Speed + Eden's Guile, but they cancel one another. Means you can use them sequentially, but why not simultaneously?

Identical effects don't stack with themselves, but overwrite - you can't get double Stunned, you can't have double Insidious Poison, you can't have double Regeneration, etc. The only exception is that some pure damage-over-time effects have code that specifically merges them with previous DOTs of the same type, such as Burning, Bleeding, and regular Poison with no added effects. Blinding Speed and Eden's Guile both give you the same type of effect, as does the Shaloren innate Grace of the Eternals, so no two of those can be giving you a bonus at the same time. (Haste gives a different status, so Grace and Haste, or Eden's Guile and Haste, can stack with each other.)

Thanks for clarification.

It would be good to have these sort of game mechanics explained in an ingame help.

Is that only for activated sources? Like, the Yeek permanent boost stacks with anything, right? Or sustains like the archmage's essence of speed (or whatever it's called)--would that stack with Grace of the Eternals or Eden's Guile?

Identical status effects only. The Yeek permaboost isn't a status effect, and Essence of Speed is called "Essence of Speed" and not "Speed" like the effect that Blinding Speed gives you.

Mouse over effects as they appear on the right. It's those labels that matter for the purposes of "duplicate" effects. For example, the Bone Armor skeleton racial and the Shielding rune are both counted as "Damage Shield", as is the use effect of the unique belt Neira's Memory. When two sources of the same buff or debuff are applied, the second tends to overwrite the first; they never stack. What you would want is to find two different buffs with similar effects (e.g., you could activate Adrenaline Surge and Arcane Destruction at the same time and get Physical Power boosts from both).