There is nothing wrong with asking friends and writers to review your book. Of course you want a decent quote on the back of the cover. These terse complimentary words of praise are called blurbs and it helps when a well known author says something nice about one's work.
However, lately I've received requests for full blown requests from people I don't know who want me to decide on the basis of a line or two. Moreover, when I politely refuse, I don't receive a word of thanks for "taking the time to consider," etc.
These books have not been offered in print, are usually unpublished, and don't contain a whole manuscript. Even if they are an ebook, I don't want to read the whole thing on-line. I expect books to be printed and sent to me.
I hate to ask a friend to do a book or a review. Done well, they are time consuming and most of the writers I know are very, very busy. But it's important to screw up one's courage and simply ask.
When I do review a book, it's usually for a publication I'm familiar with. I take reviews very seriously. Academic reviews are especially important.
Here are some of my guidelines:
(1) I read every single book I review. I don't merely skim.
(2) I never give scathing negative reviews. Books are hard to write--even bad ones.
(3) I never lie about a book, but I usually look for the things an author does well.
(4) If I don't like the genre and think the book is mediocre, I'll summarize the action and suggest that it might appeal to __________ audience.
(5) If the book is total crap I will not review it. Period. I hand it back to the editor and ask he or she to find someone else. Without much explanation.