The H. P. Lovecraft Cat Book now has a page on hplovecraft.com with a full contents list, including precise details re: the number of letters…

The Cats of New York (excerpts from 21 letters)
Old Man (longer excerpt from one letter)
The Kappa Alpha Tau (excerpts from 34 letters including “[Anthem of the Kappa Alpha Tau]”)
Musings of an Ailurophile (excerpts from four letters to Marian F. Bonner)
Extracts from Letters (excerpts from 50 letters)

So that’s 110 letters, a good haul. No Amazon listings for it, yet. Let’s hope there will be an ebook at some point, too.