What's the plural form of bookcrossing? Here's the word you're looking for.
Answer
The noun bookcrossing can be countable or uncountable.
In more general, commonly used, contexts, the plural form will also be bookcrossing.
However, in more specific contexts, the plural form can also be bookcrossings e.g. in reference to various types of bookcrossings
or a collection of bookcrossings.