Kindle Fire HD is an android tablet from Amazon for reading ebooks, watching HD movies and playing HD games. But there's a catch, you can't use it like a normal Android tab unless you root the device. This is because Amazon has locked the device to work with its Appstore. You have to buy and download Movies, Games, Books and Apps from Appstore and you can't install Google Playstore or any of the Google apps that normally work on Android devices. But there is a workaround and you don't even need to root!
Now, let's talk about reading books on the Kindle. Well, kindle devices were primarily made for reading ebooks. And you know how cool the e-ink Kindle tabs are, now add some magic to it and you get a Kindle Fire HD. But still you need to know where to get some ebooks. You can get it from Amazon but you'll have to buy them! You don't wanna buy books? Don't worry, there is a free book depot online where you can download public domain books (which are of course free)!
Meet, ProjectGutenberg !
It's the first digital library, an effort to digitize and archive cultural works and encourage distribution of ebooks. It has over 44,000 free ebooks available in various formats. The default kindle reader supports .mobi and .prc formats. Download the books you need, connect the Kindle to your computer by USB and copy the .mobi/.prc files to the Books folder of you Kindle Fire HD. Now open the books tab (library) on your Kindle Fire HD and enjoy!