There are many variants of this sauce but most of them require galangal, lemon grass, shallots, and most importantly, coriander. These ingredients are essential in Thai cuisine, these would have given the recipe a bit more authenticity.
This sauce is also called Nuoc Cham and most variations call for lime juice (the Joy of Cooking allowed a mixture of lime juice and rice vinegar), so that's what I used. I used one serrano pepper and it had the right degree of spicyness. I used two garlic cloves (less was too mild). And I ended up adding a little more sugar. I think the proportions in this recipe vary according to individual taste. All in all, it turned out great, though a bit on the salty side, but that's the fish sauce (I might use less next time). Thanks for getting me to try making this at home!