I'm reading "The 21st-Century Brain" by Steven Rose.

Why does he use an accent over the e in material?

Context suggests it is not a special use of the word material, but I could be wrong.

"Material" is a general word meaning stuff

With the diacritical, there is usu an "e" in place of the "a" and it means eqpt, apparatus, and supplies used by an organization or institution [Merriam], in other words logistical support

or, to put it another way, matériel (note spelling and pronunciation difference) is a separate (but related) word taken directly from French (meaning the equipment, apparatus, and supplies of a military force or other organization).