That’s a really subjective question. I think two very popular books are the late Bob Fenner's "Conscientious Marine Aquarist" and Nilsen/Fossa's "Reef Secrets". Personally, I think that Delbeek and Sprung's 3 volume "Reef Aquarium" is timeless and the most well written/researched material out there.
Here is a sobering thought though: there is no longer an aquarium book market. Except for self-published material, no new general marine/reef aquarium titles have been written in many years. My last book was published by TFH over a decade ago.
Forums like this began to replace books as the "go to" information resource, then social media came into play (despite the lack of quality) and now AI has been the final nail in the coffin for aquarium books.
Here is a short article I wrote about aquarium books:
https://www.amazonasmagazine.com/2017/05/01/building-a-home-aquarium-reference-library/
Here is an article where I evaluate informational sources for aquarists:
https://reefs.com/magazine/media-review-information-resources-for-home-aquarists/