The following list may give you some titles and authors to search for if you want some other possibilities. All should be available through Amazon as new or out of print.
Jump Start the HP 48G/GX Calculator: for Engineers and Scientists by Thomas Adams, Merle C. Potter, Tom Adams Paperback - 170 pages 2 edition (August 1998) Great Lakes Press; ISBN: 1881018482
Mastering the Hp 48G-Gx : A Step by Step, Easy-To-Read Introduction to Operating and Programming the Hp48G/Gx by Thomas Adams Paperback (April 1994) Kendall/Hunt Publishing Company; ISBN: 0787220892
The Hp 48 Pocket Book by James Donnelly Paperback (June 1993) Armstrong Pub Co; ISBN: 1879828057
An Introduction to Hp48 System Rpl and Assembly Language Programming by James Donnelly Spiral-bound (June 1995) Armstrong Pub Co; ISBN: 1879828065
The Hp 48 Handbook by James Donnelly ASIN: 1879828006 or ASIN: 1879828049
The Hp48 Database by James Donnelly ASIN: 1879828022
HP 48 Insights I : Principles and Programming (HP 48G/GX Edition) by William C. Wickes ASIN: 0962525855
HP 48 Insights II : Problem-Solving Resources by William C. Wickes ASIN: 0962525847
An Easy Guide to the Hp 48 : A Step-By-Step, Easy-To-Read Introduction to Operating and Programming the Hp 48 by Thomas Adams ASIN: 0840382553
Hp 48 Programming Examples (Hewlett Packard Press Series) by D.R. MacKenroth ASIN: 0201563258
Hp 48 Programming/Utilities by Loux ASIN: 9993962562