What you really want is some personal bookkeeping which doesn't make sense for the 5-star system. You'd want tags or the private note textarea to scribble down your arbitrary personal rating system.
Frankly I think more ratings are just worse. You're pitching 10 or 20 different options instead of 5.
An alternative approach:
1 - I disliked it. 2 - It's OK 3 - This is good 4 - This is great 5 - This is a must-read
That said, I think the world has also suffered from ratings inflation. I tend to assume anything under a 4 means "bad" or "meh" myself.
If you go to rate a book on good reads and hover over each of the the 5 stars, here are the "title" attributes of the links:
* title="did not like it"
* title="it was ok"
* title="liked it"
* title="really liked it"
* title="it was amazing"
I think it's interesting to have the middle/neautral rating described (3 star, middle of the range available) as "liked it" (a positive response).
Also, if you asked me about a book I read a year ago, I wouldn't be able to do much more than confirm I read it. That's part of why I use Goodreads.
1 hated it/was offended/didn't finish
2 disliked it/likely didn't finish
3 finished but it was a struggle
4 finished/enjoyed/had some issues
5 finished fast/really enjoyed/may or may not have still had issues.
If I look back at the 30 to 50 books I read in a year, it's really difficult to get any sort of ranking out of them. I've only got two choices for books I liked (4 or 5) and two choices for books I disliked (1 or 2). Having half stars would help my ratings relative to one another.