Puzzle n°37 (*) - Chewing

There is an age when young somalis "have their teeth". Just like human babies, they want to chew many things, and one of their favorites is the corner of books, including cover and inside pages ! I have a cat encyclopaedia in 5 volumes, all neatly sorted side by side like on the picture. Every volume has 400 pages, which sum up to 20 mm of paper thickness just for the inside pages of each volume,plus a nice hard cover 2 mm thick (hence a book is 20 + 2 + 2 = 24 mm thick) . One of my kittens managed to squeeze on the back of the shelf, and started to nibble very methodically all the top corners of each page, from left to right on the shelf, starting with the front cover of the first volume... As he saw me looking at him, he stopped suddenly. But he had already chewed... up to and including the back cover of the 5th volume !

On which length did he nibble in total ?

... I'm giving up ...