The preface discusses writing biographies as 'Guru service.' Does this imply that the Shri Sai Satcharita is not a comprehensive guide to all saints, but rather a personal act of devotion?

Yes, that is an accurate interpretation of the philosophy presented in the preface. Chapter 0 explicitly frames the creation of the Shri Sai Satcharita as the highest form of Guru service. The author, Balkrishna Vishwanath Dev, describes writing the biography of one's Sadguru 'by the Guru's command or by inner inspiration' as the 'best and most certain service' for attaining salvation. This service, involving writing, study, and recitation, is differentiated from the 'worldly' service of writing about politicians or even the 'partial' service of writing about many saints. The text is therefore presented as a profound act of personal devotion intended to be shared, not as an encyclopedic work on sainthood in general.