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A fairly general overview of Band-Selectable Analysis (BSA) or 
Zooming is in

  "FFT analyzers make spectrum analysis a snap," George Dovel, 
  EDN, January 19, 1989, pp 149-155.

A slightly broader view of the topic is in the following app note.
(Your local HP Sales office should be able to give you a copy of this.)

  "The Fundamentals of Signal Analysis," Hewlett-Packard Application
  Note 243.

It sounds like you would benefit from a more detailed explaination,
so you may wish to get a copy of the following article.  It describes
the digital filters used in the HP 3561A, 3562A and 3563A analyzers.

  "Hardware Design for a Dynamic Signal Analyzer," Hewlett-Packard
  Journal, December 1984 (Vol 38 No. 1), pp. 12-17.

You may also wish to buy or borrow the following book.  It has a 
chapter titled "ZOOM FFT," pp. 165-171 (but I admit I have not read it).
  
  "Frequency analysis," R.B.Randall, Bruel & Kjaer, ISBN 87 87355 07 8.

Of historic interest, one of the first uses of the phrase "Band-
Selectable Analysis" was in the following article that describes the
use of this technique in the HP 5451B analyzer.

  "Band-Selectable Fourier Analysis," Webber McKinney, Hewlett-Packard
  Journal, April 1975 (Vol 26, No 8), pp 20-24.

I hope these references help.  Obviously the articles I know about are
biased by where I work but I would enjoy reading other responses of
articles not mentioned above.  As a slight aside, the term "BSA" seems
to have fallen out of favor and most people call this "zoom" or
"zoom fft" today.

Regards,
James

--
James A. Vasil                      Hewlett-Packard Co.
Applications Development Eng.       Lake Stevens Instrument Division
jamesv@lsid.hp.com                  Mail Stop 320
Tel: (206) 335-2605                 8600 Soper Hill Road
Fax: (206) 335-2828                 Everett, WA  98205-1298