Mandie and the Windmill's Message

is the twentieth book in the Mandie series.
==Book Description==
"I love reading all the great Mandie books! I feel like I'm in all the adventures. I sometimes read a whole book straight through because I can't put them down!" writes a young reader from New York.
Mandie, Celia and Jonathan should have known that when Uncle Ned said there was a big mystery waiting for them in Holland that he meant a BIG mystery. Adventures had followed them at every stop on their summer travel through Europe with Mrs. Taft, Senator Morton and Uncle Ned, and Holland promised to be no exception. But what mystery could the quiet, peaceful land of the windmills possibly bring?
They won't be there long before they find out. Who's behind the mysterious resetting of the windmill's blades and what message are they sending? Is it a harmless prank or is someone really up to something bad? Are lives in danger?
Is someone out to destroy the royal family's reputation?
(Description from Mandie.com)