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.
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.