Adriaan de Groot's seminal work on chess thinking, "Thought and Choice in Chess" is now available as a free eBook. I found it on Google books, by doing my semi regular search for books with 'chess' as the subject. Normally this simply throws up a number of books from the 1840's which heavily plagiarise each other, but as in this case, you occasionally find a gem.
What I did not realise about this book, until I read the introduction, was that this was de Groot's first book, and was in fact his doctoral thesis. He certainly put a huge amount of work into it, as it clocks in at a heavy 450 pages.
So you don't have to go searching for it, like I did, here is the direct link to it. I'll probably try and read some of it over the next couple of months, but I'm pretty sure I won't make it all the way through.