One of the most valuable methods of book promotion is giving free copies to bloggers in exchange for their reviews. Most bloggers will only post the review on their blog, but
Keep in mind that many book bloggers only review fiction, so if you have a non- fiction title you may not be able to find too many people to review it for you. Bloggers typically review the books they are interested in and nothing else.
Another thing to remember is that some book bloggers only review printed books. If your book is only available in eBook format, you may not be able to submit to certain blogs.
Do not send unsolicited eBooks to bloggers! Most of them do not want this, and it will make some of them mad at you. Be sure to read their rules carefully before you submit. Most of them want you to send them a message first, usually with a synopsis of your book, and if they want to read it they will ask for it.
It may take quite a while before a reviewer can get around to reading and reviewing your book. Many of them have long queues of people waiting to have their books reviewed, so be patient.
One last thing to keep in mind about book bloggers is that they are not required to give you a specific review. Their review could be good or bad.
To find book bloggers in your genre, search for things like “book blog +niche” and “book reviews +niche”. There are also lists of book review blogs online.