I know you may never use socket directly, but the one time you need it, you will find out that it’s not in the book. Couldn’t it be in an appendix?
The same thing applies, to a lesser degree (for me) to the “extending” chapter.
If these chapters are available and even readily downloadable, why not leave them in the book? I can concatenate the PDFs, but i use the Mobi version a lot, and that one is a little more difficult to hack.