What contents belong in the Author's preface?

The Preface, written by author of the book can be his explanation about a concept, Writer regards is not generally known to Readers,like Milorad Pavic a Serbian Writer,doing in the beginning of his work with term:"The Khazars" -a tribe lived in VII-X / Black Sea/,a warlike and Nomadic people.The name of the book is "The Khazars /Hazards/Dictionary" The novel for XXI century/there is edition in English/ The preface can be written in a style and manner as whole book,but the writer's imagination chose for any reason to separate that,maybe emphasize. Author feels a need to explain events which coeval he was, to mention characters from real life ,kept in good and loving memory. It happens the book starts with a verse,which is not underlined to be preface but has some symbolic meaning: "Happy Years Merry Days as Torrents of Spring they gone forever." Torrents of Spring by Ivan Turgenev Probably you have seen a Gratitude to determined people,in the first page. There is second possibility to write a preface by someone else who is not Author.That belongs to some form of Critic/Review