One slightly sucky thing about a board divided into forums is that the divisions don't reflect how real conversation works. A discussion of the LCK fits most naturally into L&L (and sometimes C&C) and a discussion of the PCK fits most naturally into NOTA, but as far as I'm concerned the LCK and PCK are two halves of the same thing. There are better ways, but implementing them would involve writing my own board management software...
However, I've started reading the PCK properly (one page at a time) so it will be a while before I go back to the linguistics. Currently I'm reading the Astronomy and Geology chapter, which starts on page 38, and so far have no major complaints.
Most minor quibbles are consequences of the very condensed presentation of information. Sometimes this compactness is manifest in Mark coming down on one side of a hotly debated topic (e.g. the possibilities of planets orbiting red dwarfs, of which Mark is pessimistic), and sometimes it's manifest in segments that read kind of like revision notes or course summaries, leaving the reader to fill in the gaps. For example, on page 41 "Start with the mass M relative to the sun" would be clearer if it said "Start with the mass M of the star relative to the sun", and shortly afterwards astronomical units are introduced without actually mentioning what AU stands for. That whole section reads exactly like a refresher course, intended to remind students of things they've already been taught.
One thing not mentioned that could have been (of course, there are an infinite number of such things, so this is hardly a criticism) is the speculation that larger planets may be than our own. More generally, there could have been more of a discussion of the consequences of planets with different physical properties.
I've only found one scientific error so far. "Mars just has a couple piddly ex-asteroids orbiting it" (p44). Actually, the latest thinking is that , but this is pretty inconsequential in a book about conworlds - Phobos could have been a captured asteroid.
I'm enjoying it.
Length of year is 'p' (for period, presumably) on page 42, but 'y' on page 45. Being consistent would be better.
The section on currents (52-53) is particularly brief. (Or maybe I just say that because I studied oceanography back in the day.)
I think I've spotted a more significant fault. Starting from page 50 (in the climate section), the cardinal directions of winds are mentioned a lot but with few hints on how to generalise to other worlds (e.g. one that rotates in the opposite direction). More discussion of the underlying mechanisms would be helpful here. For example, "west" probably often means "against the direction of rotation". Related, on page 57 we read that cold deserts should be placed "in the temperate zone, shielded from rain by high mountains to the west or south". Should "south" read "equator-ward side" here?