Rollup merge of #42046 - steveklabnik:update-books, r=frewsxcv

Update the various books to latest

This includes a draft of chapter 20 of the book!

diff --git a/src/doc/book b/src/doc/book
index ad7de198561b3a12217ea2da76d796d9c7fc0ed3..97422981c53a00f7c3d6584d363443117f179fff 160000 (submodule)
--- a/src/doc/book
+++ b/src/doc/book
@@ -1 +1 @@
-Subproject commit ad7de198561b3a12217ea2da76d796d9c7fc0ed3
+Subproject commit 97422981c53a00f7c3d6584d363443117f179fff

diff --git a/src/doc/reference b/src/doc/reference
index 6b0de90d87dda15e323ef24cdf7ed873ac5cf4d3..f7a108dfa9e90b07821700c55d01f08a9adf005c 160000 (submodule)
--- a/src/doc/reference
+++ b/src/doc/reference
@@ -1 +1 @@
-Subproject commit 6b0de90d87dda15e323ef24cdf7ed873ac5cf4d3
+Subproject commit f7a108dfa9e90b07821700c55d01f08a9adf005c