Best first cookbook? As much as I would like to drop off a thick tome of technique and recipes. I think the Sunset book (or something akin to it) would be best if a used copy could be found. It covered measures and recipes in a way that was easily understood.
For someone who has difficulty with turning on a stove or oven, and doesn't know a spatula from a spoon? There was book called The Complete Illustrated Step by Step Guide to Cooking and it was very much a monkey see, monkey do, sort of international cookbook. It's about 30 years old, and can be found for a pittance. The recipes are about what you would expect, very simple, with a few needed chef notes to goose the flavor. That said a complete tyro could make a dish from it without ending up in the hospital, or burning down the house.
In the age of youtube and food television, seems that authors forget that not every budding cook is starting with a solid foundation in the kitchen. If there was a combination of the two books above with really great recipes, that'd be the single book I'd get.
I think my first cookbook was either a Betty Crocker book, or a section from Mary Margaret McBride's gigantic doorstop. Yikes, I'm old!