This paper reports a longitudinal study that observed Japanese learners in a long-term extensive reading program. Participants were eight students who continued extensive reading throughout their elementary to senior high school years. Observation periods varied between learners: they started reading between pre-school and 4th grade and ended in 11th to 12th grade. Analysis used participants’ reading history corpus and reading and teaching records which included titles of books read, how they read, notable events, learners’ comments, and teacher’s observations. Corpus analysis revealed that both reading amount (running words) and lexical input (word types) commonly increased rapidly from 7th to 9th grade. Three major phases were observed in the transition of book selection, which were partially divergent according to individual learning motivation, reading preference, and reading ability. Participants' learning motivations were found to have shifted from extrinsic toward intrinsic regulation, with some individual differences.