Is it acceptable to use working hours to read general interest books?

It's socially acceptable to do (almost) whatever you want during academic "free time". Ultimately what matters are the results you produce, and as long as you're able to do that most people will not care if you're reading general interest books, drinking coffee, or answering questions on StackExchange.

Of course, if you have assigned duties (e.g. teaching duties) then using that time to read books is not going to be permissible.

General workplace answer goes something like:

Ask your manager, and be ready to argument for whatever you plan to do. But it is manager's prerogative to specify and prioritize your workplace activities.

Academia is a special case of workplace, so asking is always a good idea. I imagine that worst case scenario would be your boss saying:

Sure, not problem, as long as that doesn't interfere with your responsibilities. And maybe not at your desk, so that people passing by have no desire to stop and chat about the book [in case of open space for example - aaaa]

On the other hand, academia is a special case of workplace so I doubt anyone would care if you read a book even at your desk. Even if it is not directly or even tangentially related to your work. As long as you fulfill your duties.

Research is such a variable area that you never know WHAT is coming down the road next. You may not need something right now, but spending some time trying to maintain a big picture when your own work is a tightly cropped photo often pays off. Reading stuff outside your own area will make you a better researcher.

If you're worried about the time you spend doing it, budget that time. For example, set aside a few hours a week for such a purpose. If you think it's important, don't skip this time. Conversely, if you think about 2 hours a week is right, don't spend three or four.