Here’s a list of great resources for additional information about all things Shinobi no Mono.
Vintage Ninja – Dedicated to old ninja movies, from Japanese silents to 80s craze exploitation, with a ton of vintage toys, comics and collectibles thrown in for good measure.
Animeigo – Tons of great Japanese films including the first four Shinobi no Mono movies.
SnM Wikipedia Page – Always a great source for information