I believe that this trick requires that you talk to a wisp and learn that the wisps are information brokers who would like you to give them any unsual information that you find.
If you take the ''Book of Lost Mantras'' from the Lyceum in Moonglow, then take it to the Wisps southeast of Skara Brae (or to any wisps in general) then they'll tell you that there's a being that they're in contact with that wants to trade for the information contained in the book. The being offers either information or gold. The wisps will advise you to take the information, which will be a complicated scientific fact. If you take the gold however, your party will be weighed down up to its maximum carrying capacity with gold nuggets, and the book stays in your inventory. You can trade these nuggets in at the mint in Brittania for thousands of gold crowns. And if that's not enough, this trick can be repeated if you find another wisp, talk to it enough to learn about the wisps' information trade again, and then afterwards exit and re-enter conversation with it.
This secret is great for equipping your party with the best equipment money can buy. Complete sets of magic armor, dozens of glass swords, hundreds of reagents, and every other sort of luxury and convenience items can all be yours without tiresome and repetitive fighting and scrounging.
Any time during the game, speak to Iolo. Type SPAM <return>, SPAM <return>, SPAM <return>, HUMBUG <return>. Do not type return, just press it. After you have entered these words, you will get access to a cheaters/debug menu where you can get items, modify party attributes and more.