MakeUseOf has 123 free eBook guides.
All eBooks are available on Pdf or/and ePub.
Use Calibre to convert non-DRM eBooks from one format to another.
http://calibre-ebook.com/.
Amazon also sell some of the eBooks.
Here's a sample of what you could download:
The unofficial beginner's guide to Tumblr.
How to start an online business.
Unofficial guide to feedly: better than Google Reader.
Take control: Android rooting guide.
Your guide to iOS7.
Tablet buying guide: summer 2013.
Cut that cord: how to ditch cable.
Get started with HTML5.
Online meeting guide: software and strategy.
Your guide to self-publishing.
Password management guide.
Switched: the convert's guide to the Mac and OS X.
How to set up your XBMC media center.
Your guide to Pinterest.
Make your own Android app: your unofficial intro to MIT App Inventor.
Your guide to what's new in Mountain Lion.
Smartphone buying guide, winter 2013.
The power user guide to Gmail.
Microsoft Office 2013: unofficial guide.
The Amazon shopping guide.
The Windows 8 guide.
Connected: your complete guide to home networking.
How to jailbreak your iPhone, iPod Touch or iPad.
Virtual currency: the Bitcoin manual.
The complete Android guide for everyone.
The complete Malware removal guide.
Bring old computer to life with Ubuntu.
The 50 cool uses for live CDs.
The PC backup guide and restore guide.
A user's guide to Calibre eBook manager.
The Windows speedup guide.
101 guide to Solid State Drives.
A universal guide to PC security.
How the internet works.
The awesome automation guide for Mac users.
How to build a gaming PC.
The internet music guide for the audiophile.
How to build a great media center PC.
The easy guide to computer networks.
The complete Twitter guide.
How to build your own PC: the easy guide.
The torrent guide for everyone.
Getting started guide to Linux.
A simple productivity guide for everyone.
The free Mac manual.
.
.
.
Definitely a bargain, especially those of you who have an iPhone 3GS running iOS 5, a windows PC running Office 2010 and you love wordpress blogging on your HP Touchpad.