My favorite book about self-realization. Osho explains everything important with perfect concision and insight, and also stays unserious which makes his perspective refreshing.
He synthesizes all the wisdom from Advaita, Kashmir Shaivism, Christianity, and Buddhism into a single view on reality that reflects the original teachings fo each tradition. This view is not meant to be taken on belief, but to be validated empirically in your direct experience.
He emphasizes that self-realization is not a crazy goal, but is a simple reality to be easily relaxed into. This is an important message for those who have been learning from spiritual wisdom, since this process often turns into an intellectual one where insights don't translate to lived experience, and the idea of Enlightenment gets cast into an unreachable ideal.