Even though book covers can be considered a form of advertising (as almost all illustration can be), When an artist refers to an 'Advertising' job, they usually referring to a specific niche of illustration, usually one dealign with magazines, film, billboards, national campaigns, music, etc.
Advertising jobs differ from book cover jobs in a lot of ways. Firstly, the budgets are usually a LOT bigger. Which is expected, since the workload, exposure, and money at stake, are all much larger.
The other difference though, and the one that often discourages a lot of illustrators from doing such jobs, is that instead of working with a single Art Director, the art approval process usually happens by committee... very large committees.
The typical result is, a lot of revisions.
And I mean a lot of revisions.
Of course, the client pays very well for these revisions, but are we really in this field for the money? For many, there comes a point when the money just isn't worth the frustration of a having a client change absolutely everything you just did for seemingly no good reason... only to then change it back.
One of the reasons for this problem is inherent in decisions made by committees. The fact is, you can't please everyone. If everyone gets their say, what you get is a mediocre mess.
(If you're ever speaking to a veteran illustrator, just ask them about their worst advertising experience. Without a doubt, they will have a nightmare story to relay.)
Take for instance the latest iteration of the 'Little Debbie' logo.
If I had to guess, the advertising agency in charge of this revamp likely spent 10's of thousands of dollars on this. They probably hired multiple artists to do something specific, then picked their favorite, and had that artist revise it until they were happy.
And what did they end up with?
Well, a logo that looks pretty much like what they had in the first place. And that's OK. It's good to revise things every now and then.
But the real question is, how many revisions did it take to accomplish this?
How many times did they send this picture back to the artist because he painted too many freckles... or he made her look 6 years old instead of 6 1/2... or her hair just didn't look juuuust the right amount of messy?
Care to guess how many times?
16 revisions, and a year to complete it.
You can read more about it HERE.