As both a writer and a reader you'd think that writing a book review -- particularly for a friend's book -- should be easy. It's not.
When friendship is involved, you don't want to sound negative even if you didn't care for the book, but on the other hand, you don't want to mislead the person reading the review before deciding to purchase the book. Talk about walking a high wire! There's no net below you on this, either.
What's a person to do?
Well, the same things you should do if you didn't know the author personally -- Accentuate the positive, allude to the negative briefly.
Why, well, because what you don't like in a book is probably entirely different from what a reader may like or not like in a book. Most readers aren't looking at what they read quite the same as a writer might. Things that act like nails on a chalkboard to you, probably sail by a non-writing reader without notice. As a writer, you're attuned to things non-writers aren't. It's like you have dog hearing and end up howling when the emergency vehicle sirens wail because, damn it, it hurts!
So stop focusing on the negative aspects, even if your teeth hurt -- metaphorically, of course. List the good points.
1) what did you like about the book, what brought a smile to your face, made you laugh, made you like or even love a character, made you want to find out what happened next
2) don't give the plot away. An editor who wrote back cover copy once told me it was much easier to do if you concentrated on the first part of the book, maybe even the first three chapters if they are long, first 40 to 50 pages if they are short. Do make reference to the plot though. The person reading your review does need to know whether this is a romance, a mystery, a quest. They will want to know where it takes place if it's a historical -- which country, what era. They might want to know what problem the main character or characters need to solve, but don't say how they solve it.
3) if it is a book in a series, make note of that in the review, perhaps mention which number book this is
4) be sure to give the author's name. Give the entire name the first time mentioned, then use their surname alone for further mentions
5) give the book's title
6) your choice on whether to add this or not: the genre niche into which it falls. This could be romantic-suspense, romantic-comedy, historical romance, paranormal romance, cozy mystery, police procedural, amateur detective, caper, spy, urban fantasy, high fantasy, political fantasy, science fiction in outer space, science fiction on a futuristic Earth or a colonized planet, time travel, alternative history, Steampunk, Gaslight fantasy...and others. If it's a young adult story or middle grade, mention that. You might even add other descriptive words to clarify the story setting and even voice. I label my own books as historical romantic mystery or historical romantic adventure, and sometimes as historical romantic mystery/adventure; as urban fantasy mystery/comedy; as Weird West Steampunk mystery/adventure with comedic touches. Someone writing a review won't necessarily use all these identifiers, but they could
7) if you don't feel the story deserves the highest review grade, don't give it one. Not all stories deserve the full 5 stars or whatever the highest mark is. Particularly if the book is by a new author, I reserve the high mark for when they have written more books, have improved at their craft and truly deserve the 5 stars. The goal of winning them can act as a carrot to the book's writer. To that end 3 stars and up (or the equivalent, depending on the site the review is posted) is still a decent rating. Personally, I never give a lower mark, mostly because, if the story and writing style merit that, I stopped reading within the first few chapters. If you don't read the entire book, don't do a review for it
8) don't put off writing the review. The closer you are to just having finished writing it, the easier it should be to remember the storyline, the presentation.
9) devote at least 100 words to the review, That's not too long and it's not too short. It supplies enough to help a prospective buyer make up their mind over whether to buy or not
10) if you have a reservation about the story, such as "not suitable for young readers", "situations would rate an R in the theatre for violence (or language or adult situations or graphic sex scenes)". I wrote in a view of a YA book that when I was the same age of the characters, focusing on a career much less focusing so intently upon a career as these characters did would never have entered my mind and thus I felt the story would appeal to readers who did have this focus rather than those looking for something more adventurous. It indicated that the story wasn't for everyone but that the correct audience would enjoy it.
I will admit that, based on reviews I've received, that I have a couple of pet peeves. One reviewer said "this appears to be the author's first book but I look forward to reading more things by her". The book they reviewed was my 24th book, the 5th historical I'd written. I was glad they'd enjoyed it but maybe an "I enjoyed this story so much that I looked up her other books and can't wait to get started reading" would have been nicer. Another reviewer said nothing about the book at all but complained about a shipping problem. They gave that problem a 1. Unfortunately, it shows on Amazon as the book being rated as a 1, not the shipping of it. But you'd never make either of these mistakes, right?
But you will write reviews for not only your friend's stories but for those of writer's you either loved or enjoyed spending time with; those you'll look for in the future when cruising the book aisle -- be they virtual or solid.