According to Ruth Harris, of LiteracyConnections.com, the best learning and most progress result from reading texts where 95%-97% of the words can be read without difficulty. She says: “If we have to struggle with more than 5% of the words, then we cannot maintain fluency and comprehension.”
To find a text that fits this guideline, she advises using the “three-finger rule.” To use this system, have the learner raise a finger each time he or she encounters a word he or she cannot read. If three fingers are raised while reading the first page (or two if there are not many words on one page), that indicates the book may be too difficult. (If you keep track of the books that prove too difficult, and reintroduce them once your learner has progressed, finding they can now read more words will produce a feeling of accomplishment!)
As you know from your own reading, interest in the content of the text is a huge aid in encouraging reading. As you and your learner get to know each other better and better, identifying potential reading material will grow easier.