I was browsing to DA recently to notice photomanipulators and digital artists in general offering their services as book cover artist.
The thing is, they really did a good job with the illustration but when they used typography, they almost ruined the image by the bad use of it. Some examples are: The texts were located with almost no margins to the covers limits, bad font combinations (like the fonts used didn´t match at all as they expresed the complite opposite), the font almost blended with the image when it shouldn´t be like that, or simply the fon´t didn´t express at all what the illustration was trying to say.
Most importanly, the use of 3 fonts or more in just ONE cover!
I had seen one with 5!! that it´s crazy! One of the basic things when making for example, book covers, it´s to use 1 2 or in extreeeemly rare cases 3 fonts for a job like that and they must be choosen and used wisely! Sometimes the use of 1 font with their visual variables (Bold, italic, condensed, etc) can really make a cover look better!Other thing I noticed it´s when people try to mix fonts, like a curly font for a capital letter and then a simple font for the rest, the thing is that if that almost 80% of the case, the mix made no sense.
EDIT: forgot to add the use and abuse on effects on the texts too, like gradients or lots and lots of fantasy fonts, like those fonts that are kind of broken/grunge looking or those with LOTS and curls and stuff.
Sometimes it wasn´t just the typography, it was how the text was placed on the image too.
Those are mistakes that I did once too and still I´m learning how to fix them, but now as I´m studying graphic design I realized how wrong it was that and how important it´s a well applied typography to a cover.
I know not everyone can afford or have the time to study a career like that, but there are plenty of tutorials that can help you with that, lots of blogs dedicated to design, lots of sites like Behance where you can see the work of proffessionals and learn by looking at what they do.
The use of a good font it´s as important as a good illustration!
Don´t never forget that.
I highly recommend not to offer a service like that if you don´t have at least some basic knowledges about this. Because it highly affects and sometimes damage the illustration work most people do. Study, see references and LEARN how to use them
Oh and by the way, this year started as a helper of the typography class in my university, that really helped a lot to notice things like that