The book 'Marked' by PC Castand Kristin Cast can be found in most well known book shops, if you can't find it on the high-street, there are many places you can buy it online - but not for free!
The reason you can't find Marked for free online as that, like films, it is illegal for people to access copyrighted content without the author's permission. Just like you and I, writers make a living by through their work - and in this case it's by selling their books.
So if everyone reads it for free, they won't be able to get any money to support themselves!
If you are keen to purchase Marked, without spending much money, then Amazon should be the first place you look into, as there are number of price options you can consider.
Firstly, there is the option of purchasing a paper copy of the book which is the most expensive option at £5.99. Second cheapest is the Kindle edition. Amazon usually have cheap deals on Kindle books, and in this case they are selling Marked for only £4.49.
Amazon also sell used books so you could pick up a copy for only £2!
Running with the second hand theme, you could try searching on eBay, as chances are someone will be looking to cash in on the book once they've already read it. Several used-book retailers also sell their books exclusively through eBay, so this is a great place to find a bargain. I've picked up books from the site for less than £1.00!!
Finally, there is also the option of looking in your local charity shop, this way way you will avoid postage and packing costs.