Cyrus Sardinus, is a mysterious figure of unknown origin. One thing that is certain that volumes of his Guidebook have been appearing for far longer than any human normally lives. It is theorised that he is a wizard, but it is also possibly that ‘Cyrus Sardinus’ is a pseudonym being used by an elf. In the past some scholars suggested that he (or she) might possibly be a dwarf, but this opinion has become extinct since his volumes have been appearing for longer than a dwarven lifespan by now too.
Obviously, his older works have become more valuable as historical works that guidebooks and have become much sought after. If you do manage to acquire a recent volume, it can be quite helpful navigating the region it pertains to. If you encounter an older work, purchase it if you can! Many scholars, especially wizards, covet them!