What's a (big) word or phrase to describe a book that is so good that I cannot put down when I start to read it?
Unputdownable has the exact meaning you described. Here's the FreeDictionary definition
“Page-turner” comes up top of mind.
Cambridge online English Dictionary defines “page-turner” as;
a book that is so exciting that you want to read it quickly.
Oxford online English Dictionary simply defines it as “an exciting book.
Merriam-Webster online Dictionary defines it as;
a book, story, etc., that is difficult to stop reading because it is so interesting
Kenkyu-sha Readers English Japanese Dictionary, the best-selling English Japanese dictionary published by Kenyusha Publishing defines “page-turner” as;
a book that is so interesting and exciting that you feel impatient to moving to the next page (turn pages).
As examples, I found the following phrase and sentences including "page-turner" and "turning pages" in the book review of Jeffrey Archer's "As the crow flies," which I picked to get an example of the use of "page-turner" at random;
- A certified page-turner - New York Daily News
- Archer plots with skill, and keeps you turning the pages - The Boston Globe
- Mr. Archer tells a story to keep you turning those pages - The Washington Post.
Besides, the New Yorker magazine has “Page-turner: On books and writing life” page as its regular columm.
When you are reading a nail biter you cannot wait to see what is going to happen next and surely can't put book down.
: something (such as a game or movie) that causes people to feel nervous because the ending is not known until the final moment