Regarding books I researched graded readers and found a few on libgen. Five from Mandarin Companion, two Chinese Breeze, and one grammar book from Mandarin Companion. All simplified by the way. Also I think there was at least one more but I got rid of it because it listed the pinyin right above the characters. To find them on libgen search by the author/editor, e.g. John Pasden or Jared Turner for Mandarin Companion. Chinese Breeze may have shown up just by searching the series name “Chinese Breeze” but I can’t remember. I’ve only read the first chapter of one of the Mandarin Companion books, but I plan to read more this summer. Regarding levels they say how it corresponds to HSK on their website I think but I can’t remember. I found one MC at a 150 character level, two at a 300 character level, and one at a 450 character level. Chinese Breeze with one at 500 words and one at 750 words.