Vintage, Rare, & Antique Books: https://www.facebook.com/groups/657123184415628

Small Press Limited Book Collectors: https://www.facebook.com/groups/193517360836341/

Fans Of The Folio Society: https://www.facebook.com/groups/60943174501/

(^ Each press typically has their own following so you can find groups for Fans of Amaranthine Books, etc.)

+ dozens of more groups.

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Maurice is working on building out a more cross platform community for collectors, still early days there:

https://www.collectiblebookvault.com/

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There is a Reddit: r/BookCollecting but I don't go there often.

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Edit:

Oh and Youtube! Booktube is a real corner of the internet and lots of discussion happens in the discords of individual creators people enjoy.