What is the correct year to use in the reference list?
You can find this in books which are published at the end of a year. For instance, if books are published in November or December, publisher will state this year as publication year in their online catalogue. But the publisher has also assigned such a published book for the following year, because it will be mainly promoted and distributed to libraries, book stores etc in the next year.  Here is another example of 2 different years of publication of a book: publisher website states 2016 and google books shows in the book 2017  In case you have read the book and not the few pages on google books, I would say the year in the book is more reliable and would cite it as 2015.