You can always purchase the book through Amazon: https://www.amazon.com/Walkers-Key-Frank-Haddleton/dp/1949066231/ref=sr_1_1?keywords=Haddleton&qid=1558180002&s=books&sr=1-1
Phoenix Books at 191 Bank Street in Burlington has copies of the book and can easily acquire more any time.
Provincetown Bookshop at 246 Commercial Street in Provincetown is stocking the book.
Support your favorite local bookstore -- any bookstore can order Walker's Key (through Ingram).
If you are in Burlington, I would be happy to sell you a copy of the book. Just call me and we'll figure something out.
I am reaching out to bookstores elsewhere and hope to arrange for others to stock the book. I will update this site as I do.