Then it will  be a piece of cake to locate "hot" books or " erotic"..etc

Or gothics :)

But you actually can pull up books by tag already. It's a bit of a pain, but if you tag a book on your shelf something you want to look for. Gothic, for example, then save, and click that tag, you will bring up all the books with the Gothic tag. Then just remove it from the book that you sample tagged, and tag that book with something else you want to look for.

You can see if there are any books already in the system with a specific tag word when you go to add the tag to your sample book. After typing the first 3 letters the tag system will try to guess what you want for a tag, and will show you anything that matches those first 3 letters. So once you type Got, Gothic will show up in the drop down as having books tagged with it, if there are any.