I just did a little scouring of Tacoma Nissan-Subaru's online database.
I can't say for sure, but if you go to
http://www.subaruparts.com/ and type in part number MSA5T9114A you get "91 Leg Supplement" and its list price is 4 dollars.
That might be the book you need, ciper.
I also found these part numbers that appear to be for some kind of 91 Legacy manuals:
MSA5T9106A MSA5T9107A MSA5T9108A MSA5T9109A MSA5T9110A MSA5T9111A
But their descriptions all get cropped out to "91 Leg Serv Man Vol." I'd guess they're probably sections 1 through 6 in order but have no way to confirm it. It's weird though that they add up to less than the price listed for the complete 1991 service manual set in their catalog. Maybe I'm missing a book or two.
"Just reading vrg3's convoluted, information-packed posts made me feel better all over again." -- subyluvr2212