imperative (n): something that demands attention or action (dictionary.com)
Basically, the idea is that I rate how strong the pull of the book is. Is it a book you'll have no problem setting down? Will it be one of those books you attempt to read whilst drying your hair and cooking dinner at the same time? The Imperative Factor of the book will tell you. Or, at least it will tell you the Factor it had for me.
For the record, I do still use the 1-5 rating system, mainly for my own
Last Resort - You're in the doctor's office. Your iPod just died, you've read the magazine in front of you twice, and the television is playing infomercials on a loop. This is the time to break out this book.
Like a Train Wreck - A very addicting book that has some redeeming qualities, but is not overall something you would recommend.
Stash in the Glove Box - Not great, but worth reading. You can easily put it down without any hard feelings for a couple weeks until the next time you need it again.
Lunch Break Read - A good book, but it's just missing that special factor that makes you feel like you never want to leave the story.
Staying in Tonight - A really good book. It's not quite compelling enough to keep you from much-needed sleep, but it's totally worth spending the evening curled up on the couch with your nose in the pages.
Stay Up 'til 2 AM - A great book. You can't tear yourself away until the need for sleep finally overcomes you, despite your valiant efforts.
Read Through the Apocalypse - A mind-blowingly, stunningly, adjective-defyingly amazing book. Nothing will come between the two of you. Come friends, dates, 4 AM, store-wide book sales, or the apocalypse, you will not be torn away.