AFAIK, you can't then print the book or download as PDF. This used to work, but the WMF decidef to break it and never fixed it.

2022(e)ko api. 16(a) 11:43 erabiltzaileak hau idatzi du ("Amir E. Aharoni" <>):

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