"'...now that I have liberated myself from officialdom, I am much drawn to the idea of using my leisure and good spirits one of these days to write a book-- or rather, a booklet, a little thing for friends and those who share my views.'
'What about?" Designsori asked with curiosity.
'Oh, anything, the subject would not matter. It would only be a pretext for me to seclude myself and enjoy the happiness of having a great deal of leisure. The tone would be what mattered to me, a proper mean between the solemn and the intimate, earnestness and jest, a tone not of instruction, but of friendly communication and discourse on various things I think I have learned.'"
That's what the book is, a friendly, wordy, gently detached communication and discourse on various things that Hesse had learned in his life.
It's an open question whether Hesse's wisdom could have been delivered in a different format with the same results.