So, in the series of “cookbooks I use all the time”, this:

As the name indicates, it’s a list of ingredients vs. possible substitutions if say, you absolutely need rice wine but don’t have an Asian shop ready. It also lists cooking method substitutions: what to do if you don’t have a claypot or a barbecue grill. It’s not exhaustive (for instance, I couldn’t find an entry for potato starch), but it’s making a freakingly good attempt at being so: the list of ingredients includes various obscure French cheeses, panko, and a lot of the Asian ingredients I often find that I have to replace at the last minute (dropping an ingredient from a Vietnamese recipe is usually a bad idea, since they rely so much on the layering of flavours to achieve their effect–remove one, and the dish kind of lacks oomph). The substitutions are pretty smart, too (even though some of them seem a bit off to me at times). But mostly, they’re smart.

The thing I use it for most, though? It’s not the substitution list: it’s the little header besides each ingredient, which lists corresponding volume and weight equivalence (ie, 1 shallot=1 tablespoon chopped shallot=15 to 30g). Pretty much a lifesaver for all those recipes which call for ingredients by weight, whereas you tend to buy vegetables by units (well, I do, at any rate).

There’s also tables listing common ingredients such as apples, potatoes, vinegars and explaining their properties. It’s less useful for me, because they’re US varieties, and for instance, the apples list has about 20-30% varieties in common with the apples I can find here. If you live in the US, I’d imagine that section would be way, way more practical.
(and I do wish there was a section on the different starches and thickeners and their uses, but fortunately Cook’s Thesaurus has a great one).