So I've been playing PF for a long time, but I'm getting that crazy "I think I'm ready to run this" feeling, and doing my usual "look at all the stuff on my shelf and throw it in a blender" style of campaign generation, the phrase "Space... The Mythic Frontier" popped into my head and all of my old Spelljammer stuff started crying to be dug out and used.
The basic idea is that a bunch of 1st level PCs are doing some adventure when a big Spelljammer crashes nearby and while the PCs are poking through the wreckage, the helm explodes, giving the characters Mythic abilities. Eventually, they'll be tooling around in a Spelljammer of their own and getting involved in interplanetary shenanigans and eventually punch Cthulhu in the face (or something like that).
So, what books should I be looking for if I want to see how Pathfinder does:
Ship to Ship combat (checking pfsrd suggests Ultimate Combat, but are there other resources)
The Cthulhu Mythos as it applies to Golarion. (I know Bestiary 4 will have the Big C.)
Space travel (I know about Distant Worlds, is there anything else?)
And as a side note - I wanted to use Beholders and Mind Flayers. Are there any good fan-made adaptations of those?
Well Mythic Adventures would be good.
Ultimate Campaign is great for every campaign.
The Core Rule Book.
You can use 3.5 Beholders and Mind Flayers, just use the guide to convert them.
Just use the 3.5 versions of MF and Beholders.
You might want to check out ultimate campaign, as they have expanded rules for quite a few things (including armies). You might want to check out book 5 of the Reign of Winter campaign, there is the troop template, which can be good for ship crews.
You might also use the tome of horrors complete, a good selection of monsters, some with an alien feel which ought to be able to shake up the typical meta game groups (nearly all experienced groups do it even if they try to avoid it) ability to predict what monsters are capable of.
Mythic adventures has a sailing ship that goes into space. It's on page 167 and is listed as a major artifact.
"These are the voyages of the Silver Maiden."
Thanks, everyone.
If you want to run ship-to-ship battles, check Fire as She Bears (FaSB). It's far better than the Pathfinder-rules. It has its flaws, but pathfinder is just silly in this area, so FaSB is still far superior.