If you are looking for specific, high-quality instruction, search for these names in digital library catalogs:

To find books specifically on Russian platforms (where your original query originated), use these terms in search engines: (Ninjutsu books PDF)

Details the tools and techniques of the Iga and Koga clans.

Communities like E-Budo or Bullshido often have "Recommended Reading" threads where users share links to free PDFs of out-of-print manuals. 3. Key Authors to Search For

While not a download site, it is the most authoritative guide for classical Japanese martial arts (Koryu), offering articles and reading lists to help you identify which books are worth seeking out.

(The Way of the Ninja download free)

For legal and free access to historical or public domain martial arts texts: