Changing margin used \addtolength with a document class "Book" applied only to header

I'm using a documentclass "book" (\documentclass[11pt,a4paper,twoside]{book}) and I want to increase the margin to be able to reduce the text width.

I've tried geometry but when using \addtolength{\textwidth}{-2cm} the command only change the headers of the pages (cf. image).

Any recommendation ?

Thanks for the support Best Jerome

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• Welcome to TeX.SX! Please help us help you and add a minimal working example (MWE) that illustrates your problem. Reproducing the problem and finding out what the issue is will be much easier when we see compilable code, starting with \documentclass{...} and ending with \end{document}. Sep 23 at 10:14
• There are more parameters that must be changed. Use the geometry package; it will properly change all the necessary parameters. Sep 23 at 11:30