The purpose of this article is to share some of my experience reading unpublished manuscripts and to provide some order to the process.
First, the terms:
A beta reader is a nonprofessional who reads the first or second draft of an unpublished manuscript.
An alpha reader does the same with an unfinished manuscript.
A proofreader is a professional who corrects syntax, spelling, and grammar.
A critique partner is a professional who assesses a manuscript’s substance and style.

For the past seven years, I’ve been reading and critiquing my friends’ unpublished, often unedited manuscripts. It’s grueling work—perusing a text for enjoyment and the author’s edification. But I do it because, well, I care about my friends. I want them to do well.
My feedback is usually forty pages of notes ranging from prologue to epilogue. My services are often not sought for proofreading, but I will note a malapropism or structural error if it’s repeated.
I start each document with some variation of the following:
I am going to really hammer at the text, so please don’t take anything personally. The book may be a fun read, but your critics and readers will not be your friends. They’ll be unforgiving, which is why I’ve done my best to be unforgiving in my notes and 100% honest about what I think are errs or flaws or places in need of improvement. Of course, everything in my notes is my perception and my suggestion, not the word of God.
I might write that you could do this, you could do that, etc., but really these are just suggestions and you shouldn’t listen to them if they contradict your vision or are just bad ideas.
I invoke the Third or Fourth Law of Neil Gaiman: “…when people tell you something’s wrong or doesn’t work for them, they are almost always right. When they tell you exactly what they think is wrong and how to fix it, they are almost always wrong.”
Also, I love strength in the simple, and I’ll look for opportunities to make your prose gorgeous, effortless.
The last thing I want is for the author to attack or argue about my notes. My constructive feedback is not a list of facts, but opinions, feelings, decisions. I recall unhappily a writer who took offense at my thoughts on a plot event and sought a debate via Facebook.

I’ll leave the details vague, since the manuscript is in its fourth draft, but basically the author wanted me to know that the event was indeed plausible and definitely not contrived. I responded that it was my job to report honestly, to hammer at anything that I felt was wrong or improbable. His response:
Yeah, no, I get that. Most of your comments are really good. But I figured you’d also not want to continue thinking a situation was implausible if it’s founded in a factual situation. Less for literature purposes and more for a general knowledge standpoint.
I doubt this author would be surprised to learn that literature doesn’t need a factual basis, that all sorts of implausible and improbable events can occur in a story without losing a single reader. Even fighting dragons can be true-to-life. My contention was the failure of the text to make the valid sound valid. Every journalist knows that the true skill isn’t reporting the truth, but making the truth sound like truth.
But I was not going to parley about prose, no matter how smug my friend’s counterclaim. A savvy critique partner will smile and move on when faced with an author’s defense.
Likewise, a savvy author will smile and move on when faced with a critique partner’s erroneous disapproval. At most, there can be inquiry or clarification. But if an author wants honest reflection, they should be emotionally ready for less-than-praise. (And read my preamble about Neil Gaiman.)
My responsibilities are to appraise a work’s style, substance, mechanics, permutations, and logic, while accounting for purpose, and to articulate my own emotional reaction, determining my degree of interest, investment, and, finally, satisfaction.

By style, I mean the way of writing. Is the methodology appropriate to the content and genre? Are there easily-fixed mistakes (like an ample use of lavish, excessive, unnecessary adjectives)?
By substance, I mean the synthesis of plot, characters, and purpose. Does the story have an engaging surface of action and suspense? Is there depth, too, and revelation about the human character?
By mechanics, I mean the text’s accordance to English language and its rules. I will usually point out a repeated mistake once with a note that the author should find the rest. I had an author whose spelling for a character’s psychic ability was “shinning” instead of “shining.” The error made it seem like the protagonist’s allies were obsessed with his legs.
By permutations, I mean the way the author innovates archetypes, tropes, and trappings. Have I seen this before? What is influence and what is originality?
By logic, I mean the internal consistency of characters and the created world-at-large. Every paragraph I read, I ask myself: Does this make sense?
Ultimately, a book should interest readers before engagement, keep them invested chapter by chapter, and leave them satisfied. My goal is altruistic. I want the book to fulfill all three elements while maintaining the author’s integrity and vision.
To reinforce the phalanx of prose by testing it with spear pokes.