Pounds and Inches: Part 2

I am interested in translating the book "Pounds and Inches" by Dr. A.T.W. Simeons. I was wondering if you would be able to tell me what paths I would need to take to go about checking its copyright issues. For eg. Do I need to be clear on the copyright issue to write a translation of the book? What if I were to add comments on certain parts to make it easier for the reader to understand certain content? Can you foresee anything else in such a task that I would have to be aware of legally? I would be deeply grateful for any information, that you could assist me with. We thought Dr. Simeon's name sounded familiar and that's because nine months ago, another one of our readers had a similar query. (See our nuanced response.) Basically, his works would still be protected under copyright but apparently his estate (or successors) are not enforcing copyright. If you add original comments to Dr. Simeon's work, you would be able to stop others from copying your original text, but of course, you could not stop the copying of Dr. Simeon's text.