diff --git a/book/src/ch03/regular_backgrounds.md b/book/src/ch03/regular_backgrounds.md
index bbcef9f..c49738a 100644
--- a/book/src/ch03/regular_backgrounds.md
+++ b/book/src/ch03/regular_backgrounds.md
@@ -285,4 +285,17 @@ it will loop.
 
 ## Mosaic
 
-TODO: talk about mosaic.
+As a special effect, you can apply mosaic to backgrounds and objects. It's just
+a single flag for each background, so all backgrounds will use the same mosaic
+settings when they have it enabled.
+
+The mosaic control is a read/write `u16` IO register at `0x400_004C`.
+
+There's 4 bits each for:
+
+* Horizontal BG stretch
+* Vertical BG stretch
+* Horizontal object stretch
+* Vertical object stretch
+
+The 
\ No newline at end of file
diff --git a/book/src/introduction.md b/book/src/introduction.md
index a202d07..27a0d80 100644
--- a/book/src/introduction.md
+++ b/book/src/introduction.md
@@ -51,7 +51,7 @@ folks in the server as well (there's a few hundred folks).
 If you want to read more about developing on the GBA there are some other good
 resources as well:
 
-* [Tonc](https://www.coranac.com/tonc/text/toc.htm), a tutorial series written
+* [TONC](https://www.coranac.com/tonc/text/toc.htm), a tutorial series written
   for C, but it's what I based the ordering of this book's sections on.
 * [GBATEK](http://problemkaputt.de/gbatek.htm), a homebrew tech manual for
   GBA/NDS/DSi. We will regularly link to parts of it when talking about various