Cooking guide for individual ingredients (not full recipes)?

Practical answer: If you want to know how to cook ingredient X, Google 'X recipes' and you will usually find at least some very simple recipes. Unless baking is involved (where most of the ingredients serve a critical chemical purpose and are in fairly precise ratios) you can ignore the parts of the recipe you are not interested in and extract the basic information of how to cook X. For example, this will be fine without the bacon and chestnuts, this is pretty simple already, and this will be fine without the herbs.

Less practical answer: There are cookbooks that will have this information, but the ingredients described will differ by the individual cookbook. For example, I have The Roasting Tin by Rukmini Iyer, which includes roasting times for various ingredients at the start of each chapter to encourage readers to make substitutions depending on what they have. I don't know the range of ingredients you're interested in, so I don't know if that book covers your needs.

Older cook books were based on this principle. I have my old school cookbook and my mother has the one she bought when she was just married.

Likely you can pick one up second hand, if you prefer paper over internet. These are all rather thick books and the entries for the different items will be rather short.

You will have to allow for a change in taste. The book my mother has cooks all veggies till they are soft, online sources will mostly keep quite a bit of bite to them. Depending on how things changed in your area, there might also be changes about preparing meat. Or a lot of the things now for sale may not be in.

I use mine mostly to work out how long the different parts of the meal take, so they are all done at the same time.

A very traditional, UK, book is mrs Beeton's. Contains everything they could put their hands on back then, but it is quite a while ago.

There are many different types of cookbooks, beyond the recipe collections which dominate the market, so maybe there are some guides of this type (like Willeke, I have seen books which contain such information, although I don't recall one where it was the main content). But there is no specialized term for them, so I don't think you can do any kind of focused search for them.

One reason for the rarity is that this information is usually learned the other way round. Learning the skill of cooking is, in large parts, learning techniques for preparing food. So people (especially in professional schools) tend to learn technique-by-technique. And part of the knowledge about a technique is also the knowledge about the food categories for which it is applicable. For example, when you learn how to make starch-based puddings, you will ideally also learn that the pudding technique works with any kind of ground grains, but does not work with other kinds of food.

Since there is a rather limited list of techniques that are in common use, you end up with a list of techniques in your head, and the knowledge what kind of food can they be used on, and the recognition in the other direction happens more or less automatically. So, in your example, a chef would look at brussels sprouts, and if wondering what to do with them, a list of good options will come up spontaneously. It probably won't be a full list of all suitable techniques known to the chef, that's why creativity is prized among chefs; but it is pretty sure that no unsuitable options will present themselves. If the chef selects "roasting" from this mental list, then there will be no need for them to look up a recipe - they already know how to roast vegetables in general, so they will go on and roast the brussels sprouts.

So, in the long-term, you will best improve your skills by learning techniques one-by-one. It is a gradual process, so at the beginning, you easily supplement it with exactly what you have been doing, looking up individual recipes. If you trust books more than random Internet hits (which is not a bad heuristic in itself), then just get yourself a few books specialized in the directions in which you are likely to want to cook, and have fun exploring them.