Below are curated lists of online resources for materials, tools, instruction and history.

John Neal Bookseller: Bookbinding and calligraphy supplies located in Greensboro, NC
Hollander's: Bookbinding materials and tools located in Ann Arbor, MI
Affordable Binding Equipment: The website title says it all. Jim Poelstra is an experienced cabinetmaker who also makes excellent (and affordable) bookbinding equipment such as presses, cradles, sewing frames, and more
Talas: Bookbinding & conservation supplies located in Brooklyn, primarily geared towards conservators and conservation labs
Hiromi Paper: Located in Culver City, CA, they are an excellent resource for Japanese papers and tissues, as well as selections from around the world
Nanami Paper: Fine stationery, materials, and tools from Japan, located in Irvine, CA

AIC's Book Conservation Wiki: The online guide to techniques, treatments, materials, and technologies used by book conservators, put together by members of the AIC Book & Paper Specialty Group
Philobiblon (Book Arts Web): They have an extensive resource page, as well as all the issues in PDF of Bonefolder the journal for book arts
DAS Bookbinding: Provides a nicely organized and wide-ranging series of instructional videos
iBookBinding: Bookbinding tutorials and resources
/r/bookbinding: The bookbinding subreddit