This is an endlessly open question, and much depends on where you are in your cooking journey, and what styles of food you like to cook. I can only describe the way I went about it.
I got a love of cooking about 20 years ago when my wife and I moved into the first house we had with a usable kitchen. I wanted to preserve my own family's tradition of Sunday dinner (which in England is a roast), and the only way that this was going to happen was if I cooked it.
So, given that Sunday dinner was the primary target, I got a copy of a book that focused on British classics. It happened to be Gary Rhodes'
New British Classics. Fast forward, and I decided to try vegetarian Italian stuff (my wife was a vegetarian at the time), and got Ursula Ferrigno's
Truly Italian. In both these cases, I was not buying a book with a hodgepodge of all sorts in it, I was focusing on one cuisine, and learning a few recipes from it. Ferrigno's book was the first one I cooked from front to back, and I learned a lot of basic skills like risotto, building soups, getting pasta right etc. For French, I looked to
Raymond Blanc as a Frenchman cooking in England with access to British ingredients.
Fast forward again, and I wanted to really learn stuff and develop my own dishes. For that, I use a core library of
Larousse Gastronomique for general recipes, French food and authenticity, I use Escoffier's
Le Guide Culinaire for ideas (cooking from that takes some effort),
Institut Paul Bocuse Gastronomique for techniques and basics, Anna del Conte's
Gastronomy of Italy for Italian, and finally I cook a bit of stuff from the
Roast Chicken and Other Stories series by the well-regarded British chef Simon Hopkinson (those
books are written according to ingredient and season).
Of those books, the Simon Hopkinson ones were actually a good bedtime read because that and his
Week In Week Out are actually columns from the UK's
Independent newspaper. The
Roast Chicken books are the books I would 'carry around' as they are small paperbacks and a great read.
Finally, there is the Internet. I uses this to work out how to do specific things. The most reliable sites for me are
BBC Good Food (for ease) and
Serious Eats (for furiously researched amazing stuff).
Chef Steps is good for working out Sous Vide timings. Ian Haste's
Haste's Kitchen and Simon Quilter's
Food Busker YouTube channels are also inspiring places to get ideas. But the Internet changes all the time, so these things come and go.