I am learning Photoshop at home, but I am finding it quite difficult.
Can anyone recommend me any book(s) or resources to learn it (preferably for free)? I have a good internet connection but I'm not sure where to start.
The edit to the question now makes this answer seem a bit "off". The original question was "What is the best book to learn Photoshop?"
PeachPit Press' Adobe Photoshop Classroom in a Book is the book written by Adobe. It covers every single feature and tool in Photoshop and is the definitive source for any information regarding how the application works. It's even better than the Help files in many cases.
That being posted, be aware... it is not exciting. It is not compelling. It is not a book full of tutorials or "how to" information. It is essentially technical manual describing a tool/feature and how it works. There will be no description as to when or why you would use a feature, merely how the feature functions. It is basically a written version of the Help files with more care taken to editing and accuracy.
If you are looking for "how to create x" type of things, you'll need to refine your track. There are great books on digital prepress, photo retouching, web design, painting, etc. But they are generally restricted to that track and don't really stray.
Welcome to GD.SE! Since you are new to Photoshop, you could check these similar questions, I'm sure you'll find lots of useful information in them (they sound rather specific to programmers, or to web, but they actually have resources for all kinds of design):
I personally found design magazines quite useful when I had just started, most come with some tutorials and good examples of what the current trends are. Perhaps you could subscribe to something like Advanced Photoshop. Nothing beats online tutorials, though, they are way easier to follow (and they usually come with starting materials).
I recommend you this book : http://www.amazon.com/Adobe-Photoshop-CS6-Classroom-Book/dp/0321827333. It explains you every tool and how you can use it. Everything is step-by-step and pictures are helping you to understand easier. You can buy it for kindle or in paperback format.
PS Basics :
Oreilly Missing Manual PS CS6... & more: http://it-ebooks.info/tag/photoshop/
The Book Of Pixel Perfect Principles: http://cdn.ustwo.co.uk/PPP/PPP2.pdf
Adobe Photoshop CS5 on Demand Sampler Book: http://www.slashdocs.com/kpzyky/cs5-on-demand-sampler.html
Hair Masking using Refine Edge In Photoshop: http://www.photoshopforphotographers.com/pscs5/downloads/Movie-08.pdf
Keyboard Shortcuts & CheatSheets :
Photoshop Unknown Tricks & Time Savers: http://www.smashingmagazine.com/2010/01/20/obscure-adobe-photoshop-time-savers/
Best Tutorial Sites (Also search in Google, Youtube, Vimeo,Lynda etc) :
PS Contests :
Sketching, Manipulation, Photography: http://www.pxleyes.com