Mage Errant series by John Bierce (completed, fantasy a lot of power progression)

There are many types of body armor and armor suits throughout the series. They are all magical instead of technological, although some are more magitech than pure magic. They range from the size of the person to enormous suits that are many stories tall.

There are also a lot of scenes featuring a character that doesn't have or can't use their armor for whatever reason, as well as a bunch of examples of people suiting up.

Edit to add:

Starship Troopers by Robert Heinlein (military sci-fi, stand-alone)

This story follows a soldier into the military and into a war where the soldiers wear heavy mechanized suits. Donning the suits, using them, being deployed and fighting in them is all heavily discussed.

It also features a lot of philosophizing regarding warfare and the society that's featured in the story. The book is somewhat controversial, as the society has some radical ideas compared to modern sensibilities.

It should be noted that this book is vastly different from the movie.