More than just a Pretty House
Bookcases are serious offenders when it comes to clutter. Either the books multiply, and gain friends along the way such as fashion magazines, or we are putting all the things that do not have a defined place on the shelves. As such, revising the contents of them should be a habitual task.
If you find that you are unable to keep your bookshelves tidy, then schedule time each and every month to revamp those pesky harbourers of bric-a-brac, and junk. First line up all the books properly, and eliminate stacks of floating papers, coupons, childrens drawings and whatever else has migrated there. Then, find a new place to collect items that are not decorative in nature, and that only subtract from the overall appearance. Move the stuff there and make it the official home of all banished unspecific items.
Why we seem to forget that our bookcases are not concealed from view remains a mystery, but it does seem to be a common problem. As such, just pencil in the act of refreshing the space, and then all will be right once again. Otherwise, you just become another victim of the unsightly bookcase syndrome.