The catalog, titled "Impressed By Light: British Photographs from Paper Negatives, 1840–1860", is priced at $75 and is available in hardcover only (members get 10% discount) - if you buy from the Met. However, I found that Amazon.com has it for $47.25. Also, if you buy this book/catalog from the Met along with the book "All the Mighty World: The Photographs of Roger Fenton", they have it for $95.00.
I hope they will have a softcover edition of the catalog sometime in the near future.
Please be sure to call the museum to check that all four galleries are open before you visit. I believe the gallery that was closed yesterday was the one that had photographs from India and Burma, titled "Under an Indian Sky" as per the Met's website. I was really interested in this gallery.