There's really only one thing I can't do without:
I am completely an utterly incapable of going anywhere without a book. I can't even go to the bathroom for fear I will get stuck in there for a good deal of time ("thinking" as my mom would call it). This might seem strange to some, especially when I am in the lab and leave my backpack but bring my iPad (which has my eReader on it) and a winter coat (because it's about fifty degrees in the woman's bathroom!). It can also lead to some hilarious and frantic searches as you are trying to hold it in but can't find your chosen reading material.
I had one time where my car broke down when I was only doing a quick run to the grocery store. I had to stay by my car and was stuck there for over an hour. Because I wasn't planning to be gone long, I didn't bring a book with me. I had some nasty things to say to, fingers to fling at, etc the other drivers and the occasional pedestrian. I was about pulling my hair out with boredom. And that's hardly the only example.
I've also had plenty of instances where I forgot to bring a book to work (back when I worked at Labcorp) or finished my book before the day ended and didn't have anything else to read.
I. Would. Go. Mad.
Just like I can't function on too little sleep, I have to be able to fill my lull periods with reading. I need a book. I need to be able to read. Twitter doesn't cut it. Facebook doesn't cut it. Playing sudoku and puzzle games don't cut it. I need my books.
If you're wondering what brought this on, I just got stuck without a book. I don't think it was very long, but I didn't have a watch on me and it felt like an ETERNITY.