Along with Amazon, Goodreads is essential for me in determining if a book is worthwhile. Goodreads also helps sort out a book's publishing history. Then I buy from the store with the best price, often Kobo with their discounts. A high percentage of 5-star reviews eliminates a book from consideration as fast as a low rating average.
Since I am obsessive about finishing the books I begin, I never begin a book until I've learned all I can about it and am justifiably hopeful that I will enjoy it.