The best resources we have are our teachers. Direct transmission, from Long Sensei to us, is how our Koryu art is learned. From teacher to student, from Sensei’s heart to our heart (Jikiden).
Some resources can help with concepts and visualisation when combined with direct teaching and learning in the Dojo.
Samurai Swordsmanship: The Batto, Kenjutsu, and Tameshigiri of Eishin-Ryu
This is an excellent book written by the late Masayuki Shimabukuro, Hanshi and 21st Soshihan of MJER (Masaoka-Den), and his senior student and successor, Carl E. Long, Hanshi and 22nd Soshihan of MJER (Masaoka-Den).
There are also two DVD sets by Shimabukuro, Hanshi and Long, Hanshi. The first is a three volume set on Samurai Swordsmanship: (Link to DVD 1 below)
There is also a set of three DVDs on Advanced Samurai Swordsmanship:
Finally, there is the classic book: Flashing Steel, by Shimabukuro, Hanshi, and Mr L Pellman. Iaido Gibraltar would recommend the Second Edition over the first and third.