Geekiest.Net in association with Tradepub, providing you a huge collection of Magazines, White papers and podcasts absolutely free of cost.
Here are some interesting ones
Your Unofficial Raspberry Pi Manual – This free guide can tell you everything you want to know about Raspberry Pi
Smartphone Buying Guide 2014: 7 Things You Need to Know
iPhone Photography, from Shooting to Storytelling
Laptop Buying Guide 2014: 9 Essential Tips
Wiley's Windows 8 Book Bundle -189 Page Sampler - Windows 8 tips, tricks, and insights
How to Make Windows 8 or 8.1 Look and Feel Like Windows 7
HackerProof: Your Guide to PC Security
And many more
If you are an XP user who is going to upgrade to a newer operating system, you can find some helpful books on Windows 7 and Windows 8, 8.1 Tips, Tricks and how to, which will come handy for you. like The Windows 7 Guide: From Newbies to Pros, Windows 7: Tips & Tricks, The Ultimate Windows 8 Kit etc.
It also includes guides for Mac OS, Linux, Ubundu, iOS, Android. Which covers Security, Tips and Tricks etc.
Check the complete range of books here Free Tech Whitepapers