You seem to be talking about two different types of stylists (there are more than just these two types, by the way)
 
1. Personal stylists (sometimes called a personal shopper) .... who often work for a company like a big department store or or high end retailer, helping customers put together outfits. They are primarily sales people ... the goal is to sell more clothes to the customers by providing special customer service.
 
There are also personal stylists that are just hired by someone to help them shop and pick out things ... or just shop for them. These stylists work for the customer ... not the store.
 
2. Freelance fashion stylists and fashion editors in magazines ... who both work with photographers to produce fashion editorials in magazines. Fashion Editors work for the magazine ... freelance stylists are usually brought in to do a specific editorial shoot. They work with designers to borrow what they need to create the looks they need ... so they become a very important person to fashion designers because they can provide free publicity. 
 
Freelance fashion stylists also work for various designers and retailers in creating the company "look" for ads, lookbooks and catalogs. Fashion editors would not work for anyone other than the magazine.
 
I'm afraid that there are not a lot of resources about freelance fashion stylists ... it's a fairly new "career" which didn't exist about 20 years ago. But there is a ton of information about that job in a few threads right here on the Fashion Spot. You will have to wade through a lot of information that might not be applicable to what you want to know ... but you will be able to get a better idea about how it all works, if that would help.
 
Stylists vs. Personal Shopper
A Day in the Life of a Fashion Stylist
Stylist & Photographer: working together
Fashion Stylists: Clothes and Connections
All About Becoming a Fashion / Wardrobe Stylist ( I think I saw some recommendations in this very, very long thread, about books about styling .... you may wish to search the thread using book and books as keywords to find them.)
 
 
 
And a little bit about how Celebrity Stylists work (in case you have an interest in this type of stylist) .... those stylists who are hired to dress the stars.
How Celebs Borrow Gowns, Etc. from Designers for the Red Carpet
About Being a Celebrity Stylist