After my learning curve with unity3D, I started looking into the 2D world… The first question in my mind was
What is the best 2D Game Engine?
I found the answer at slant.co with unity 3D leading the race….unity 3D game engine for 2D development?… You should be kidding me. 
With the release of Unity 4.3, there are now native 2D tools in the engine.
Unity is a 3D engine, right? Not quite – now it’s more. It comes with a dedicated and streamlined 2D workflow. You’ll be surprised to discover quite how flexible and resourceful a single spritesheet can become.
Our 2D tools make importing sprites, populating scenes, adding physics and animating so easy. What’s more, with one exception, all our new 2D features are currently available in the free version of Unity!
-Unity3d
Getting Started
Download Unity 4.3 here.
When you first open up Unity 4.3 and create a project, you are presented with a new default option for 2D as a drop down below the package import selection area.
Give an appropriate name to your project (Replace New Unity Project 1).
By default, it is set to the 3D. Once you make this selection and start your new project, the scene will automatically be setup for 2D.
Unity is a project-based application. In practice, this means every time you make a new game, you’ll make a new project. In Unity, 1 project = 1 game.
You’ll notice there is now a 2D toggle in the scene which basically locks the camera, sets it up for you and makes it incredibly easy now to start building a 2D game. You can go back to 3D at any time by simply unselecting the 2D button.
Now you have successfully created a unity 2D project….What’s next??
Either create your own 2D game or walk-through an existing game to learn about sprites, animations, 2D physics, scripting, etc.
Hands-on
Download the demo project and take a look through it today!
Go to search dialog (next to create) and type unity 2d-> select assets store in the dropdown.
Pseudo Code
If (you are registered with unity)
{
click "Import package"
Open Scene (under the main Assets folder)
In Unity, a level (or an environment) is termed a scene. 1 scene = 1 level.
}
else
You know what to do.
Books
The best book, in fact the only book in the market as of today is:
Learn Unity for 2D Game Development By Alan Thorn
From Sprites to 2D components, this topic specifically contains content for 2D game development.
