Unfortunately there is no way in-game to get another copy of the Nericon book. If you read over the 'lost items' procedure a couple times but still have a problem with it, let me know where exactly and I'll try to explain it more clearly.
Though a bit overwhelming at first glance, if you have any experience moving files around on your computer (organizing stuff or making backups, etc) it shouldn't be too hard to replace the book. If you save under a new name before starting, and go through the procedure step by step, it is unlikely you could make any mistakes that aren't easy to recover from.
You do need to speak to the merchant Marcus about the letter from the cave. He should direct you to Tingalf the black market dealer, who gives you a hint about where his goods are coming from (don't think you need that hint, though). If you head to the Rivertown sewers entrance in the middle of the Ars Magicana square, then go a bit east, you'll find some thugs. The leader drops a letter you need to take to Lt. Robin (usually in the north west part of the square). Once Tingalf is arrested, you should be able to tell Trevor that to complete the quest.
Marcus-Tingalf-Trevor-Quest
