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.
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.
This book was not part of the original set 1-4, but seems to have been produced later with the same cover style and telling solid triangle color on the top and bottom of DJ spine.