People with a massive book collection how do you organize your bookshelves?

So my dad asked me to help him organize his books with him. He has a massive collection that he bought over the span of 50+ years probably contains atleast 200+ books ranging from all sort of genres, from politics to fiction to religion to non fiction to culture etc.

I am not sure what's the best way to do it. I thought about categorizing by genres but his collection contains so many different genres I don't think it will be the best method.

I thought to ask here for any tips since I reckon some people here might have gone through something similar.

Not sure if the mods will keep the post since it's technically not about a specific book but rather how to handle them, so sorry if I broke the subreddit rules