It was always one of my favorites when I was growing up in the early 70s - the heyday for Sesame Street and Sesame Street merchandise. The plot is pretty simple - Grover is convinced there's a monster at the end of the book (hence the title) and he is trying to convince the reader not to get to the end. The more pages you turn, the more frantic he gets. This is a spoiler free post, so you're going to have to discover the ending for yourself.
I bought a new copy before Jackson was born and it has remained in heavy bedtime rotation ever since.
Bonus tip: to take the reading experience to a whole other level, you have to read it in Grover's voice. I find a build up of phlegm helps but of course, your results may vary.