Hi,
I would imagine you could go to the Cisco Press site and buy the eBook. I have not bought any myself. I have gotten the previous books through courses I have taken.
http://www.ciscopress.com/markets/detail.asp?st=44730
I only just started getting the certifications myself so I have just done CCNA R&S and will probably go on taking CCNA Security.
Personally I will probably prefer the actual book than an eBook/PDF
- Jouni