From the announcement:
The Shilka Publishing Digital Reinforcements program is now live. The idea is simple. Every Shilka paper book now has a code included in the back. This code can be used to get a digital version of the book for free.
eBooks and paper books each have advantages. Paper books don't need a special device to read on, but you can keep an entire reference library of eBooks on a phone or eReader. For many people, the ideal solution would be to have their books in both paper and eBook editions. However, the cost of buying the same book twice puts people off – far better to spend the money on another book!
Shilka customers can now read their books in paper and digital formats, taking full advantage of both, at no extra cost. The free eBook can be downloaded in several formats, so that it can be read on any device: Mobi (for Kindle devices & apps), ePub (for other eReaders & eReader apps), and PDF (for reading on a computer). eReader apps are available for all computers, tablets and smartphones.