Hi,

As far as I can see, the Collection extension, which provides the Special:Book page, is deployed on nearly all Wikimedia wikis.

Is there data that shows how often do people actually use it?

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Amir Elisha Aharoni · אָמִיר אֱלִישָׁע אַהֲרוֹנִי
http://aharoni.wordpress.com
‪“We're living in pieces,
I want to live in peace.” – T. Moore‬