See existing tags when adding new ones

When entering tags for a book, it would be useful to see the tags you've already entered. This has the advantage of making it easier to add tags (just clicking on ones you'd like to apply via ajax and/or a checklist) as well as helping to prevent entering multiple tags that basically mean the same thing (e.g., funny vs humorous). That would help (me, anyway) from fragmenting my books among a bajillion tags/keywords.