On Chapter 11: x86 Assembly Language Programming https://www.freebsd.org/doc/en/books/developers-handbook/x86.html There is a broken link for "Jeff Duntemann's Assembly Language Step-by-Step" This one seems more appropiate: http://www.duntemann.com/assembly.html
That link also appears to be broken now. I have replaced the link with just plain text, including the ISBN number, and the user can pick their favourite book store.
A commit references this bug: Author: allanjude Date: Mon Sep 28 22:10:42 UTC 2015 New revision: 47454 URL: https://svnweb.freebsd.org/changeset/doc/47454 Log: Remove a broken link from the developers handbook PR: 202249 Submitted by: pfg (original) Approved by: bcr (mentor) Sponsored by: EuroBSDCon DevSummit Changes: head/en_US.ISO8859-1/books/developers-handbook/x86/chapter.xml