For science writers, the most important advice I can offer is this: please take the time to make sure your book is free from errors. Many standard thermodynamics textbooks are loaded with them, and they only serve to confuse students. On the other hand, it takes an almost unimaginably huge effort to create an (almost?) error-free science book. For my own thermodynamics book for instance, which is only 184 pages long, plus the additional content for the website, the total effort required was something like 2000 hours. To be fair though, in my case a lot of extra research was required, in order that my book serve as an effective supplement for ~70 or so standard thermodynamics textbooks across various disciplines.