These complementary resources may be helpful for taking advantage of the material in the KLC and KLC Graded Reading Sets series:
A Dictionary of Japanese Grammar, by Seiichi Makino & Michio Tsutsui.
All About Particles: A Handbook of Japanese Function Words, by Naoko Chino.
Exploring Japanese Literature, by Giles Murray.
Genki: An Integrated Course in Elementary Japanese (Level 1), 2nd Edition, by Eri Banno et al.
Genki: An Integrated Course in Elementary Japanese (Level 2), 2nd Edition, by Eri Banno et al.
The Handbook of Japanese Adjectives and Adverbs, by Taeko Kamiya.
The Handbook of Japanese Verbs, by Taeko Kamiya.
Japanese Sentence Patterns for Effective Communication, by Taeko Kamiya.
The Kodansha Kanji Usage Guide, by Jack Halpern.
Making Sense of Japanese, by Jay Rubin.
Speak Japanese in 90 Days, by Kevin Marx.
Tobira: Gateway to Advanced Japanese, by Mayumi Oka, et al.