Based on a longitudinal study of 10 young Japanese learners who continued reading books for 6 to 14 years, eventually progressing to reading paperbacks for L1 teenagers, this study conducted a lexical and non-lexical analysis focusing on the books they used during their elementary school years to find out why they found certain books difficult. The data used in the analysis were taken from Reading AtoZ (levels aa to H) and Oxford Reading Tree (stages 1+ to 7), which they used until they moved on to graded readers for adult beginners. Reading AtoZ level aa books were mostly covered by known vocabulary and vocabulary they could associate with the illustrations. As the levels progressed, the number of words that were difficult to associate with the illustrations increased. It has been found that the books often reported difficult are due to lexical factors, irregular syntactic structures and unknown contextual backgrounds.