A long time ago, '+' was used as a special tag to indicate a book marked as read in your library. After the introduction of custom columns, it became much simpler to use a custom column to indicate a read book. If you refer to the sticky on creating catalogs that I directed you to in an earlier post, you'll see an example of using a custom column for read status.
Here's a
link to a post describing the change in 0.7.32. Note that you can still use '+' as a tag to indicate read status, but it's not nearly as useful as creating a custom column so you can know when you finished reading the book.
G