What is the correct year to use in the reference list? I am wondering what would be the correct year to use for this book?:

http://www.worldscientific.com/worldscibooks/10.1142/9195

On the page it says 2014, and on google scholar I see that other people have used 2014. But inside the book as can be seen here

link,

it says copyright 2015. What year should I put for it?
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.