Sorry if this has been asked and answered already but will this book contain generic/real world deities for homebrew game use? Campaign Setting books generally don't have things that are friendly for out of Golarion lore but the description "Details on nearly 300 deities from across the Inner Sea region and beyond." sounds promising.
I believe the book focuses on in-setting deities, i.e. the BIG 20, with a little information on some others. But all 300+ (gods - including racial - & demigods/demon lords+nascent ones/empyreal lords/archdevils+infernal dukes/horsemen+deacons/qlippoth 'almost-lords'/elemental lords/eldest/etc.) that have some link in the present to the Inner Sea region, only, should be appearing on some multi-page mega-table, or so I think I've heard...
Outer gods possibly also included in the table. (Don't know about great old ones, 'though...)
So no Azlanti gods. Nor Thassilonian. Nor ancient Osiriani. Nor those from Tian-Xia, Casmaron, Vudra, Arcadia, Sarusan or Southern Garund. Unless they're worshipped in the current age in the Inner Sea region.
There are probably no Psychopomp Ushers (according to J.J.) listed. Nor asura ranas & rakshasa immortals (Vudra/Casmaron), kami lords & oni daimyo (Tian-Xia), or kyton demagogues (for similar reasons that there might not be any ushers...? <shrug>). Probably. But if there are, yay!
I'm pretty sure the book focuses on those in-setting gods & demigods already published to date that are linked with the Inner Sea region at the current age of the setting.
So, unless said god is currently worshipped in the Inner Sea, real-world or 'generic' (What's that?) gods are unlikely to be featured in the book.
But you never know...
Cue Paizo Employee to confirm/ deny any/ all of the above - or not.
::EDIT:: & Ninja'ed by the one of the principal authors himself! ;p