Biased but generally good.
I'm going to ding just one star, but for multiple reasons. Normally, I'd be a lot harsher, as the errors could easily have been avoided and there's no excuse for that. In this case, though, none of the errors really change anything. The historical facts are essentially correct, the chain of cause and effect in the decypherment is fundamentally sound, the errors are purely ones of bias, with touches of insufficient research in places. A single consequential failing equals a single star of penalty.
It's clear the author is highly caught up in Ventris' brilliance (and he was brilliant) and the failings of centralised, proprietary research (a known problem in academia). Both points are valid, although it should be pointed out that centralised researchers are capable of brilliance and that being independent doesn't make one brilliant. It's a complicated world.
The new-fangled "s's" is annoying but in style guides and grammar texts. I dislike it intensely as it is disruptive to smooth reading and worsens the correlation between written and spoken English.
Some of those who played a crucial role in decypherment are given insufficient acknowledgment and credit. That matters in this game we call academia.
Unfairness isn't merely a matter of who gets to see their name in print (many involved are dead, in any case), because of the way research is evaluated (and funded) and because of how libraries with academic sections try to figure out what is important enough to keep on the shelves, significant work that rarely gets referenced rarely gets seen. It's similar to excessive secrecy in the effect it has, it's very toxic. I have no desire to reward an author for the fact that everyone gets fairly treated across the literature as a whole, the author had no say in what others did and played no positive role there. There being no real harm done is happenstance and that's not a good thing to promote.
Nor did he invent the practice of not being secret. Archimedes and Euclid relied extensively on circulating ideas and documents being updated, preceding him by quite a bit. He didn't really develop the idea much beyond his strategy of releasing the worknotes early-ish and often. For real progress in this conceptual bazaar of ideas, you'd need to wait a decade or so and into a wide range of sciences.
However, as I said, these do not change the events or the processes. These are peripheral to the main story the book is about, which is the cracking of Linear B. This is not an instruction manual on decrypting languages, nor is it professed to be anyone's story other than that of Michael Ventris. It certainly isn't claiming to be a book on grammar! They are technical errors, not factual errors.
I never recommend buying just one book on how something was discovered, as omissions and contextual weirdness is inevitable. This is no exception. If you want to understand a discovery, get into the minds of the discoverers, mentally journey with them, you need both enthused narratives like this (nobody can learn from the dull) and objective but lively books.
Always start with the enthused, if you can, as you need to be interested to go further. This is a great book as your starting point on Ventris-spotting. If that's all you want, that's fine. If you're intrigued, as I am, on intellectual problems and challenges, this is only your starting point.
I will wrap up by saying that the three major breakthroughs in ancient texts (hieroglyphs, cuneiform and Linear B) all used very different methods. Yet other methods exist today. If you're eager to be Ventris Mk. II and solve the remaining unreadable languages, you have a chance but expect your reading list to have some really odd titles.