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.