Kraken handbook
From Kerbal Space Program Wiki
The different types of krakens
There's already too much Krakens in KSP, but many are left to uncover. That would be more interesting to research and use them, rather than avoid them. — Nerem Kerman, Kerbal Wizard of the KSC
The Kraken Handbook contains knowledge about the Krakens and their uses.
Summoning a Kraken
I don't know any way to do this, see Youtube or browse on the net.