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?

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