What to call them depends on use; "tags" in scientific or other publications are more often called keywords (but are used as Index Terms) and are often the actual topics or important words in an article rather than meta tags. For example Barack Obama is a good keyword, politics is more like a tag or category.
Tags are handy in that they give a visual metaphor for the action of "tagging" something, just like you're sticking a little piece of paper on whatever content you're marking up. I believe this is part of why Tag has become the most prolific term within more web-savvy users.
Categories is a good term if only one or few tags are going to be used to apply to an item, especially when the tag sorts or filters the content
Labels is easily the most relatable term as labeling things is common in real life. You label your CDs, label your lunch at work, it just makes sense. If there's no better semantic fit for the term, label is probably the way to go.
For a tooltip it of course depends on which term you use and more importantly how the user will be using the tag. If tags are to help other users find your content, tell them that. If tags are to help the user find their own content, tell them that; one might tag items differently for themselves as opposed to others, they may have their own vocabulary that makes lots of sense to them, but are meaningless to others. Explain what the tag is doing, don't try to explain the whole concept of meta content.
As per your update I feel the most clear term would either be categories or sections, to borrow from bookstore terminology. Sections isn't a conventional way to refer to tagging content but it is logical in the book world as bookstores and libraries are often divided by section.
Unfortunately section does imply a physical divide, which may be less helpful if each book belongs to a great deal of specific tags; however it does make perfect sense to say you're browsing the "French" section and thus only French books appear, doesn't it?. It does have the plus of feeling very bookwormy however, which depending on your application could be some simple but logical fun.