What's a word or phrase to describe a good book that I cannot put down when starting reading?

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?

"Page-turner" is common. I've heard people say "potboiler," but i believe this inapt, as "potboiler" originally meant a work churned out merely to earn some lunch-money.

Unputdownable has the exact meaning you described. Here's the FreeDictionary definition

The phrase compelling read is often used to describe a book or article that is so good that one is drawn to finish it.

engrossing, absorbing, captivating, gripping, fascinating, riveting, consuming, addictive read

“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.