I have a couple of cookbooks that are now very well worn. I think the best of them is probably Appetite by Nigel Slater. It's a little unique in that he treats his recipes more as suggestions rather than rules. He encourages lots of experimentation --- after all, everyone's taste is different, so you should make something
you like instead of something the author likes. Each recipe comes with a handful of variations so you can see how you can start to build new dishes out of ones you already know. There's a great section at the beginning that goes through lots of herbs and spices and suggests combinations of ingredients that generally work well together. But maybe the best thing about the book is just the writing. It's hard not to read it and not start to appreciate food.
If you're strictly interested in techniques, I like Keys to Good Cooking by Harold McGee. If you're brave, you can read through his magnum opus, On Food and Cooking, which goes through all the science. Keys to Good Cooking is sort of the cliffs notes version and just tells you what you need to know. But there are no recipes in either.
Then there's Joy of Cooking. It's an old standby and it's hard to go wrong with it. There's tons of information in there.
Lastly, look into anything by America's Test Kitchen. It's sort of the polar opposite of Nigel Slater's approach. They test every recipe, sometimes ~100 variations, to try to find the precise recipe that they think works best. If you want a recipe that you know will just work, it's probably the best place to start.