Is there a manual for this game? Where can I get it?

I've done a quick search and it doesn't look like there is one, probably because this game is too new in date and spirit.

Most games replaced the manual by in-game tutorials and outsourced the production of actual guides to forums and Youtube videos long ago. Sadly game manuals are considered as a legacy, annoying waste of time by most developers (I don't agree with this, but it's the reality we face).

I recommend you to look up for user guides like this one:
https://steamcommunity.com/sharedfiles/filedetails/?id=1576663819

And to watch Youtube videos for the visual how-to.

It doesn't look like there is a wiki for this game, but for many times there is one where the game concepts are explained.

Another probable reason why this game lacks a guide is because it was both marketed as indie and inspired by the UFO remakes, so the developers probably expected the niche gamers to already know how it was played.

This game combat mechanics is taken form the XCOM and XCOM 2 (not to confuse them with the original UFO games from 1993 and 1994) games, you could also check if those have a proper guide.