Nowadays it's not uncommon for authors to reach out to their readers and ask their followers to help make their work the best it can be. Because of this, I thought I would draw up a little post listing the various stages of editing. If your author knows what they're doing, this is what they will expect from you, should you answer their call to:
Beta Read/Content Edit: You are the author's first line of defense. The document is likely a hot mess because only the author has worked on the book. If their are any plot holes afoot or anything wonky with the storyline in general, this is where you can help find it. You can bring up typos and grammatical errors and such but it will likely be a waste of your time. You're working on an early draft and what you try to edit might never see the light of day.
Line Editing: You're working on the final draft, and you really shouldn't be doing this part unless you're an English major. A line edit is an in-depth process. Your job is not only to catch errors but to fix syntax as well. It is your responsibility to make sure that the author is saying what they mean. Clarity is the point. Because of this, line editing can create just as many problems as it fixes. You're rearranging words, swapping them out for better ones, juggling prepositions and slaying vague antecedents. Spaying dangling participles. Furthermore, if you choose to read a book that still needs to be proofread, you should know that there will likely still be numerous errors. This is not because the author has a crap line editor. It's likely because they have a good one. Which brings us to...
Proofreading: You are the author's final line of defense. While I advise you have only one line editor, you should have several beta readers and proofreaders. Imagine proofreading as the finish on a piece of furniture. The design was drawn, the pieces were cut and molded and sanded and assembled, and now they need a sealant. If there are problems with the plot, if there's something you don't like or feel shouldn't be there, oh well. Seriously. At this point, if your author knows what they're doing, they've already had a content edit and paid for a professional to line edit the piece. Any major changes will cause more errors. If your author is mad because you didn't point out their major plot hole, once again, oh well. Tell them to find a better content editor. Proofreaders catch typos and other errors left in or created by line editing. It's as simple as that. If you're reading a galley, you might want to mention problems with formatting as well.
Reading an ARC (Advanced Review Copy): This can get tricky. Some authors, myself included, send out ARCs before proofreading is complete. Usually while proofreading is going on. Your job isn't really a job. Your author is seeking a review. Nothing else is expected or required.
I hope this helps to clarify the roles of each job. If I failed to mention something important or you have any questions, I'll be in the comment section
*hugs and high fives*