I've searched around the meta, but I haven't yet been able to find any consensus on this. Is it okay to post a feature-request asking for a specific book and/or author to have their own tag?
Once you have 300 reputation (which you already seem to have) you can just create the tag yourself, by adding a completely new and previously unexisting tag to your (or any other appropriate) question.
If you don't have the necessary reputation yet, just leave a comment below the question and other people will eventually create the tag for you. Or you might even flag a moderator to do it, but for a newly created question, a comment is more than sufficient as it will be seen by many people anyway.
For works that don't have any question yet, there shouldn't be a tag anyway.
So as to your particular question, no there is usually no
feature-request needed for this. If you think there are already many questions existing that would deserve a specific new tag, then this might be appropriate for a
feature-request, as it can also be debated by the community this way.
So much to the general question of creating tags. As to the particular question of creating tags for individual works, I'd refer you to DarthSatan's comment:
It's not necessarily appropriate for every single individual book to have it's own tag. Consider if the objective can be met just as well by an author tag, or a tag for a series (if a series).
So if you're unsure if a certain tag should be created or not, asking a meta question about it might not be a bad idea.