The following articles (and, perhaps more useful to you, author names) were published on the June 1991 issue of the Hewlett Packard Journal:
HP 48sx Scientific Expandable Calculator: Innovation and Evolution by William C. Wickes, Charles M. Patton
The HP 48sx Interfaces and Applications by Diana K. Byrne, Robert W. Jones, Patrick J. Megowan, Gabe L. Eisenstein, Ted W. Beers
The HP 48sx Calculator Input/Output System by Steven L. Harper, Robert S. Worsley
Manufacturing the HP48sx Calculator by Richard W. Riper
Hardware Design of the HP 48sx Scientific Expandable Calculator by M. Jack Muranami, James P. Dickie, Preston D. Brown, Mark A. Smith, Lester S. Moore, Thomas B. Lindberg, David L. Smith
The following articles also appeared on the same issue, and may (not sure) be related to the HP 48s development:
HP Solve Equation Library Application Card by Eric L. Vogel
Improving the Product Development Process by Douglas Daetz, William P. Carmichael, Edith Wilson, Spencer B. Graves
I hope it helps