This is a very good question, but an easy one to answer. You can put them anywhere. It doesn’t have to be a formal place. Just share your reviews.
Where You Purchased Your Book
Most online retail sites will let you write a review. Most are such like Amazon, Barnes and Noble, and iTunes. These sites like to have reviews as they encourage sales on their site. Reviews help consumers make their choices which helps these sites make money.
Give back to the site where you got your book. Let me know what you thought of the book.
Your Own Website
There is always the option of putting reviews on your own website. You can control what your review looks like and get credit for any and all traffic. And you can have fun with it.
You can write your review any way you want to. It can be funny or serious. You can have pictures or not. It is all what you want it to be. Enjoy putting your book reviews on your own site.
Share your reviews in newsletters. Let your followers know what books you are reading and what you thought of them. You don’t have to put the whole review in the newsletter. Just include a snippet and a link. Give the reader a hint of your review. If they are interested to know more, they can follow your link.
These typically are tied to websites. If you don’t have one, link it to your Facebook page where you talk about books.
For Someone Else
There are bloggers and other sites who would love to have book reviews. They’ll welcome others reading the books sent to them and giving them reviews for their sites. I once had a site I read for, but I burned the bridge due to personal issues that delayed my reviews. Be careful with that. Think of yourself as an employee having to meet deadlines.
There are sites set up just for sharing the love of books. They focus on book reviews and links to purchase the books. They might also have services to help an author get the word out about their book. Many readers use these sites without thinking of their full purpose.
One very popular site is Goodreads. This site has been around for years and has been a wonderful place for readers to connect with each other and with their favorite authors. A reader can leave a review that is either just a star rating or write out a full review. There are contests to win free copies of books and events that bring readers more reading excitement.
Reedsy is another site I discovered recently. Sign up to become one of their reviewers. You can produce a written review or one in video format. I have found some wonderful new authors from this site.
No matter where you put them, write reviews. Even it is only a star rating, share what you thought about a book. A review goes a long way for an author. Reviews get the attention of other readers and spread the word about a book.
Create your own site. Write for others. Share the wonderful world of reading.