diff --git a/book_format.py b/book_format.py
index 0c459b7..100dd51 100644
--- a/book_format.py
+++ b/book_format.py
@@ -12,7 +12,7 @@ sys.path.insert(0,'./code') # allow us to import book_format
 
 def test_filterpy_version():
     import filterpy
-    min_version = [0,0,20]
+    min_version = [0,0,21]
     v = filterpy.__version__
     tokens = v.split('.')
     for i,v in enumerate(tokens):
diff --git a/pdf/merge_book.py b/pdf/merge_book.py
index 59c6dd1..88d5027 100644
--- a/pdf/merge_book.py
+++ b/pdf/merge_book.py
@@ -45,7 +45,7 @@ if __name__ == '__main__':
          '../08_Nonlinear_Filtering.ipynb',
          '../09_Unscented_Kalman_Filter.ipynb',
          '../10_Extended_Kalman_Filters.ipynb',
-         #'../11_Particle_Filters.ipynb',
+         '../11_Particle_Filters.ipynb',
          '../12_Smoothing.ipynb',
          '../13_Adaptive_Filtering.ipynb',
          '../Appendix_A_Installation.ipynb',