diff --git a/.github/workflows/book-build-container.yml b/.github/workflows/book-build-container.yml
index 1e012f7b87..bca6372f7b 100644
--- a/.github/workflows/book-build-container.yml
+++ b/.github/workflows/book-build-container.yml
@@ -440,14 +440,11 @@ jobs:
       - name: 🧪 Preflight toolchain (Linux)
         if: matrix.platform == 'linux' && matrix.enabled
         working-directory: ${{ vars.BOOK_QUARTO }}
-        env:
-          # Layout: <repo>/mlsysim/ is the project dir; <repo>/mlsysim/mlsysim/
-          # is the actual Python package. Put the project dir on PYTHONPATH so
-          # submodule imports like `mlsysim.core` resolve without an editable
-          # install. Parallels the fix in book/tests/test_units.py.
-          PYTHONPATH: ${{ github.workspace }}/mlsysim:${{ github.workspace }}
         run: |
           set -eu
+          # In GitHub's Linux container jobs, $GITHUB_WORKSPACE is the in-container
+          # checkout path. The github.workspace expression is the host path.
+          export PYTHONPATH="$GITHUB_WORKSPACE/mlsysim:$GITHUB_WORKSPACE${PYTHONPATH:+:$PYTHONPATH}"
           echo "🧪 Running Linux toolchain preflight checks..."
           run_check() {
             CHECK_NAME="$1"
@@ -468,6 +465,7 @@ jobs:
           run_check "python3 on PATH" "command -v python3 >/dev/null"
           run_check "python3 version" "python3 --version"
           run_check "mlsysim import" "python3 -c \"import mlsysim,sys; print('mlsysim:', mlsysim.__file__); print('python:', sys.executable)\""
+          run_check "book tools import" "python3 -c \"from book.tools.figures import style; print('book.tools.figures:', style.__file__)\""
           run_check "mlsysim.core import" "python3 -c \"import mlsysim.core; print('mlsysim.core:', mlsysim.core.__file__)\""
           run_check "Rscript on PATH" "command -v Rscript >/dev/null"
           run_check "Rscript version" "Rscript --version | sed -n '1p'"
@@ -594,11 +592,10 @@ jobs:
         if: matrix.platform == 'linux' && matrix.enabled
         working-directory: ${{ vars.BOOK_QUARTO }}
         env:
-          # See preflight comment: <repo>/mlsysim/ must be on PYTHONPATH so
-          # Quarto's embedded Python can resolve `mlsysim.core` during render.
-          PYTHONPATH: ${{ github.workspace }}/mlsysim:${{ github.workspace }}
           CHROMIUM_FLAGS: "--no-sandbox"
         run: |
+          # Keep the in-container checkout first on PYTHONPATH for render-time imports.
+          export PYTHONPATH="$GITHUB_WORKSPACE/mlsysim:$GITHUB_WORKSPACE${PYTHONPATH:+:$PYTHONPATH}"
           # Ensure Quarto can find Chromium (installed by Dockerfile but .bashrc not sourced in CI)
           if [ -f "$HOME/.local/share/quarto/chromium/chrome" ]; then
             export QUARTO_CHROMIUM="$HOME/.local/share/quarto/chromium/chrome"
diff --git a/book/quarto/config/_quarto-pdf-vol1-copyedit.yml b/book/quarto/config/_quarto-pdf-vol1-copyedit.yml
index ba35c5df9b..f228c6f2e8 100644
--- a/book/quarto/config/_quarto-pdf-vol1-copyedit.yml
+++ b/book/quarto/config/_quarto-pdf-vol1-copyedit.yml
@@ -117,7 +117,7 @@ bibliography:
 
 execute:
   env:
-    PYTHONPATH: "../..:../../mlsysim"
+    PYTHONPATH: "../..:../../mlsysim:../../../../..:../../../../../mlsysim"
     MPLBACKEND: "Agg"
 
 metadata-files:
diff --git a/book/quarto/config/_quarto-pdf-vol1.yml b/book/quarto/config/_quarto-pdf-vol1.yml
index b0484952da..8116d5ebee 100644
--- a/book/quarto/config/_quarto-pdf-vol1.yml
+++ b/book/quarto/config/_quarto-pdf-vol1.yml
@@ -138,7 +138,7 @@ metadata-files:
 
 execute:
   env:
-    PYTHONPATH: "../..:../../mlsysim"
+    PYTHONPATH: "../..:../../mlsysim:../../../../..:../../../../../mlsysim"
     MPLBACKEND: "Agg"
 
 editor:
diff --git a/book/quarto/config/_quarto-pdf-vol2-copyedit.yml b/book/quarto/config/_quarto-pdf-vol2-copyedit.yml
index 0af88fc4c2..ca4e739a3c 100644
--- a/book/quarto/config/_quarto-pdf-vol2-copyedit.yml
+++ b/book/quarto/config/_quarto-pdf-vol2-copyedit.yml
@@ -119,7 +119,7 @@ bibliography:
 
 execute:
   env:
-    PYTHONPATH: "../..:../../mlsysim"
+    PYTHONPATH: "../..:../../mlsysim:../../../../..:../../../../../mlsysim"
     MPLBACKEND: "Agg"
 
 metadata-files:
diff --git a/book/quarto/config/_quarto-pdf-vol2.yml b/book/quarto/config/_quarto-pdf-vol2.yml
index fd2b7e54f1..d0b62e1e2a 100644
--- a/book/quarto/config/_quarto-pdf-vol2.yml
+++ b/book/quarto/config/_quarto-pdf-vol2.yml
@@ -141,7 +141,7 @@ metadata-files:
 
 execute:
   env:
-    PYTHONPATH: "../..:../../mlsysim"
+    PYTHONPATH: "../..:../../mlsysim:../../../../..:../../../../../mlsysim"
     MPLBACKEND: "Agg"
 
 editor:
diff --git a/book/quarto/config/shared/base/execute-env.yml b/book/quarto/config/shared/base/execute-env.yml
index 01874d6cf2..54f7623227 100644
--- a/book/quarto/config/shared/base/execute-env.yml
+++ b/book/quarto/config/shared/base/execute-env.yml
@@ -1,6 +1,6 @@
 execute:
   env:
-    PYTHONPATH: "../..:../../mlsysim"
+    PYTHONPATH: "../..:../../mlsysim:../../../../..:../../../../../mlsysim"
     MPLBACKEND: "Agg"
 
 editor: