If you're a user of LinkedIn, you'll know that it's a work-related social network that can be very useful as a way of finding a new job or some new customers.
In terms of numbers, it has roughly the same number of subscribers as Twitter, so it's a significant player in the market. And it's now owned by Microsoft.
To get the best out of LinkedIn you need to understand how it works, and you need to have a really good personal/company profile on there. If you don't have one, but you'd like to, then there's a useful book called LinkedIn Profile Optimization For Dummies. It runs to around 330 pages and contains a lot of useful tips. And while it normally costs around $14, you can get it for free as a PDF file for the next week or so.
Just head to https://www.tradepub.com/free/w_wile254/prgm.cgi and fill in your details, and the download link will be sent to you. The e-book is then yours to keep. The download itself is around 22 MB, and is malware-free according to VirusTotal.