As a writer, you have to become expert at dealing with constructive criticism since it is aimed at helping your book become better. When choosing someone to read your book, ask them for feedback, whether it be positive or negative, but remember that any constructive feedback is better than nothing.
What's the Difference?
There is 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 your 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 negative input.
Constructive criticism looks like this:
"I read your book, and highlighted some sections that just felt off to me. I also corrected some grammar mistakes."
Criticism looks like like:
"Your book was probably one of the worst books I've ever read. I wouldn't even bother sending it to a publisher."
Notice the difference between the two? You might've also noticed that this criticism looks quite a bit like some book reviews you've seen online. While it's difficult, you should learn to just ignore this type of negative feedback.
The Value of Good and Bad Criticism
Readers who give you constructive, helpful criticism are treasures. Keep them close and treat them well so that they will be willing to help with other projects. Be sure to be appropriately grateful for all help provided by these individuals.
You will encounter criticism during your writing career — it's inevitable. How you react paints an interesting picture of who you are. If you publicly react in a negative way, potential readers may be turned off by your unprofessional behavior. However, if you respond respectfully or just plain ignore these types of comments, you will end up benefiting from the bad experience.
No Matter What...
Be sure to thank people for reading your book, regardless of their comments. Remember that they didn't have to take time out of their busy schedule to read it and let you know what they thought.