diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 346d50f8..a6802d32 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -28,7 +28,7 @@ jobs:
           # Note: Specifying the version of valgrind and wasm-pack is not supported.
           # cargo-watch is supported by cargo-binstall (through quickinstall)
           - cargo-hack,cargo-llvm-cov,cargo-minimal-versions,parse-changelog,cross,nextest,shellcheck,shfmt,valgrind,wasm-pack,wasmtime,mdbook,cargo-watch
-          - cargo-hack@0.5.15,cargo-llvm-cov@0.4.11,cargo-minimal-versions@0.1.5,parse-changelog@0.4.9,cross@0.2.4,nextest@0.9.11,shellcheck@0.8.0,shfmt@3.5.1,wasmtime@0.39.1,mdbook@0.4.20,cargo-watch@8.1.1
+          - cargo-hack@0.5.15,cargo-llvm-cov@0.4.11,cargo-minimal-versions@0.1.5,parse-changelog@0.4.9,cross@0.2.4,nextest@0.9.11,shellcheck@0.8.0,shfmt@3.5.1,wasmtime@0.39.1,mdbook@0.4.21,cargo-watch@8.1.1
           # Nextest supports basic version ranges as well
           - nextest@0.9
         include:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 89f46a63..d44057a0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ Note: In this file, do not use the hard wrap in the middle of a sentence for com
 ## [Unreleased]
 
 - Update `wasmtime@latest` to 0.39.1.
+- Update `mdbook@latest` to 0.4.21.
 
 ## [1.5.8] - 2022-07-21
 
diff --git a/main.sh b/main.sh
index 15178bf7..f5617d5d 100755
--- a/main.sh
+++ b/main.sh
@@ -269,7 +269,7 @@ for tool in "${tools[@]}"; do
             ;;
         mdbook)
             # https://github.com/rust-lang/mdBook/releases
-            latest_version="0.4.20"
+            latest_version="0.4.21"
             repo="rust-lang/mdBook"
             case "${OSTYPE}" in
                 linux*) target="x86_64-unknown-linux-gnu" ;;