Yes, some of them are useless (especially some of the modern ones) but some are priceless. I love the Grandmother's Best series that I first got a few of free and have sent bought others; the women who wrote them is the grand daughter of a lady who trained as a hotel chef in the late 1800's and wrote professional cookbooks herself when she was younger.
The worst ones seem to be those books cobbled together from Internet sources by people trying to make a quick buck with an e-book. Then there are some by people who may be good cooks, but don't really understand how to communicate recipes or other similar information very well.
A good way to weed things out, if you are in a hurry, is to look at the customer reviews. You don't even have to read them, just look for stars if you are really rushed. Between 3 and 5 stars means at least one person probably found it useful; and since it says how many people reviewed it, you can see if 45 people rate it an average between 4 and 5 starts it is probably worth it for free or even 99 cents.
If it has one or two stars and more than one review, it is probably not worth it even for free. Anyone can dislike a book, but if more than one person dislikes a cook book for good reasons (like the one that said "all the recipes in the book are from the Inter, book is poorly written and contains many errors") just skip and move on.
I am hoping to put my Barn Cat book up, but I can say in my blurb that all the information has been reviewed by a Vet which it has. People writing animal care books who are not professionals should always try to have this done as a matter of course, or at least get a qualified vet tech to review the material before publishing.
expatriate Californian living in rural Ireland with husband, dogs, horses. garden and many, many cats