I came aross a book: http://www.uesp.net/wiki/Skyrim:A_Minor_Maze, depicting a maze that is supposedly in the Labyrinthian. I've seen screenshots or something of it elsewhere on UESP (can't find it now), but I can't find the actual maze anywhere in-game.

Did it get cut or something?

It is there- it is found topside of http://www.uesp.net/wiki/Skyrim:Labyrinthian_%28place%29.

its refering to shalidor's maze, a fun little "side quest" in labyritheon.

http://www.uesp.net/wiki/Skyrim:Shalidor%27s_Maze - I keep forgetting to go there and look for it.

Yeah.... where?

There's Labyrinthian proper, to the west that's quest-locked, and then there's another to the northeast that's not ("Lost Valkygg"). Haven't found the maze...

Also, I just got the Konahrik mask and... I'm not finding any sign of the Summon Dragon Priest spell/power. Checked Conjuration, checked greater powers, checked shouts...

it's hard to explain but if you thouroughly explore the outdoor section of the labyrinthian ruins you should find one door that leads to shalidor's maze.

as for the mask, i thought konahrik's only perk was if your health is low, it gives a chance to fully heal you while damaging those around you.

edit - ah..on the mask, it's a expert level spell so maybe your conjuration skill isn't high enough for it to show?

I'm nearly at Master on Conjuration. I've been siccing my dremora lord on boss-in-mook-leveled-creature-list Draugr Deathlords left and right. I've been cutting everything else apart with my atro-forge-sourced daedric swords.

But no conjure dragon priest spell...

You don't receive a spell to cast dragon priests. There is a chance mask will summon one automatically when your health is low like the other effects

It's in Labyrinthian.
You go to labyrinthian in Arena too!