I've found the book I mentioned. Details:
Title: HP-71 BASIC Made Easy
Author: Joseph K. Horn
Publisher: SYNTHETICS
ISBN: 0-9612174-3-X
It's a very enjoyable, little book, intended for those
people who've got an HP-71B and know next to nothing
about its functionality. It uses a very intelligible style,
and guides the reader effortlessly, step by step, from
the basics of CALC mode all the way to LEX file creation,
with lots of humour, very good examples, and detailed
explanations.
Even the most difficult topics are dealt with painlessly
and with considerable detail, such as handling of all
types of data files, and advanced uses of PEEK and POKE.
It also includes references for all functions usable in
CALC mode as well as a comprehensive Quick Reference Guide,
which includes functions and statements for the Math ROM,
among others.
Finally, the gem of the book is the chapter dedicated
to LEX files, which includes the incredible MAKELEX
BASIC program which allows the user to enter LEX files
from the keyboard on a bare bones HP-71B. The author also
includes a number of interesting and useful LEX files,
which can be entered that way.
All of this on a short, very amenable book. It must
be very difficult to get a copy nowadays, but any user
or collector interested on the HP-71B would benefit a lot
from it, it will change your appreciation of the machine.