As a writer, you should be an expert at taking constructive criticism since it only helps make your work even better than what it could have been. When you pick someone to read your book, you should be prepared for feedback, whether it be positive or negative, but remember that any constructive feedback is better than nothing.
What's the Difference?
There is, however, a difference between constructive criticism and just plain criticism. Constructive criticism may point out a flaw in a book and might even provide some helpful tips on how to fix the problem. The person giving constructive criticism wants to help you and work work succeed, so they will give feedback for your personal benefit. Criticism on the other hand poses as feedback when it is actually just name-calling and putting down.
Constructive criticism looks like this:
"I read your book, and I think it could use a little work. I highlighted some sections that just felt off to me and I also corrected some grammar mistakes."
Criticism looks like like:
"Your book was probably one of the worst books I've ever read. It needs a ton of work before you even think about sending this to a publisher."
Notice the difference between the two? You may also notice that criticism looks quite a bit like some book reviews you might see. They are vicious attempts at tearing down a book that may have no merit whatsoever. These individuals might even be under the impression that they are being helpful, but do not have the appropriate tact be considered good at constructive critics.
The Value of Good and Bad Criticism
When you come across someone who gives constructive criticism well, keep them on your radar in case you have future projects, and you might even be able to help fellow writers in pointing them in their direction. It should become a mutually beneficial relationship and you should take time to nurture that relationship.
It's inevitable that you will encounter some criticism in writing career, but how you react to it paints an interesting picture of who you are. If react in the same fashion they do, potential readers may be turned off to that unprofessional behavior and not read your book. However, if you either respond respectfully or even ignore it, it will not have any negative impact on your book. Just as how you would respond to this person would reflect on you, think of how their behavior makes them look: rather poor indeed.
No Matter What...
No matter what, be sure to thank people for reading your book because they didn't have to take the time out of their busy days to do it. They didn't have to take the time to let you know what they thought about it and what you could do to make it better. No matter what happens when someone responds to your book, act like the professional you are and don't let it get you down.