I've seen it enforced on a very random basis- usually with the new "freshman" FA and the very senior FA as well. The "freshman" usually take the book and put it in the overhead and as soon as they get chimed at 10k they jump up and apologetically return it to the owner, whereas the senior FA just demand that it gets stowed for Taxi/takeoff/landing.

At my carrier, we have pouches for water bottles and "novel" sized paperbacks which eliminates the issue somewhat- except that nobody ever uses them for anything other than trash.