2000-06-15. Harry Potter and the Sorcerer's Stone Illustrative Style Guide.
The image of the invisibility cloak (back cover) came from Warner Brothers' Illustrative Guide; the phoenix (front cover) is from a public domain engraved icon that appears in the late 18th century/early 19th century.
Stylistically, it is clear that this edition was intended to extend the four-book set, cf. SET-TUR-HP1~4-03859. The books were apparently individually numbered (copyright pages indicates this although the one exemplar we have is missing the numbered page for an unknown reason) and they are stylistically, identical.