There is an unfortunate ambiguity involving the sequence and names of the Spider World series.  

The 1987 edition of "The Tower", as well as the 2001 one, include the text of what would otherwise be the first three volumes of this series: "The Desert", "The Tower" and "The Fortress", in that other.  Volumes 4 to 6 are "The Delta", "The Magician" and "Shadowland", in that order.

Therefore, the 1989 edition of "The Tower" is not equivalent to either the 1987 or the 2001 editions, but instead the middle part of both.  Those reading it will want to read "The Desert" first and will need to bridge their reading with "The Fortress" before following into "The Delta".