I've found the Ash Guardian Tome, and I'm guessing you get the Ash Spawn Tome from the same place (Neloth/Neloth's apprentice).

But where are the Conjure Seeker and the other Tomes? I can't seem to find them in a vendor anywhere. Do you need to go somewhere and pick them up?

You find the Conjure Seeker tomes inside of Apocrypha.

Where in Apocrypha?
Are they just random loot on the dead Seekers?

No, they tend to be on those tables that you always find a few books on. That's where I always find them.

I've never found one on the table things.
Damn, I guess I'll have to go and backtrack in Apocrypha...

You can also purchase it from the apprentice. If his inventory does not change, just save and re-load.

Ive few new summon spells. Can someone say all of them names so i can see have i them all

Conjure Seeker
Conjure Ash Spawn
Conjure Ash Guardian

I think there's one more, i'm not sure...

When I got the first book from Raven Rock Mine I found the seeker spell tome on a table very close to where you first teleport in close to a black pool.