John, et al:
On the use of "in the Spirit" as a structural marker (actually, I prefer the
term "linguistic" marker), see Bauckham's _Climax of Prophecy_ (which, by the
way, also has a very good outline of the book) or my dissertation "The War
in Heaven". There are some good works out there dealing with an understanding
of "in the Spirit" in Revelation. Some references may be found in chapter 2
of my dissertation, but here are some that I can remember:
1. Merrill C. Tenney, _Interpreting Revelation_, Eerdmans Publishing, 1951.
Once you get past Tenney's preterist approach, some of his discussions are
quite fascinating. The material on _en pneumati_ is in his introduction (if
I remember correctly).
2. Bauckham, _Climax of Prophecy_, I think his first article in this book
deals with this issue. Again, I am going on memory. This book contains
several very good articles regarding the structure and theology of Revelation.
3. David Aune's Word Biblical Commentary on Revelation has a discussion of
this formal marker in the introduction as well. I'll have to check for
pages, but I do remember that it is in volume 1 of the commentary.
4. John M. Court, _Myth and History in Revelation_, John Knox, 1979. This
book has an article that deals with the visions associated with the phrase
_en pneumati_. I think it is around page 140 in the book.
These are the four books that I remember off the top of my head dealing with
this idea of _en pneumati_ as some kind of marker or as an ecstatic or
prophetic category. I'm sure Bauckham and Aune could give many more (and
probably do in their works).
I hope that helps!