I've been focusing on soup making the past few months, and have done lots of research on good soup cookbooks(I currently have 7) to have and reference.
What I have come to learn is stocks and broths are the soul of a soup. In order to make a world class soup, you must take the time to make your own stock/broth. I have two books just on this subject:
Broth & Stock by Jennifer McGruther
Mastering Stocks and Broths by Rachael Mamane
The recipes are great in both of these. My eyes have been opened to the various stocks that can be made. We all know beef(brown) stock, chicken stock, fish stock, and vegetable stock. I have learned how to also make roasted garlic stock. Mussel stock. Fire Roasted Tomato stock. Parmesan Broth. A wealth of recipes in these two books.
I will leave you with three 'hints' to make a great stock/broth:
1. Always skim the liquid for fat/residue
2. Wait to add aromatics
3. Be careful to ladle out the broth from the pot instead of pouring.