There is a brand new guide book available for camping in Bulgaria. It is called Camping on Three Seas (The Black Sea, The Adriatic and The Aegean).
The book covers campsites in Bulgaria, Greece, Turkey and Croatia.
The book is available through many retail stores and most campsites in Bulgaria and costs 8 Lev (just under 4 UK pounds).
It is available via online order as well but have not tried this.
The book is very nicely put together but it is only in Bulgarian language at the moment. However the book is clearly laid out with icons showing the services at each campsite, prices in Bulgarian Lev etc. There is a fairly self explanatory legend at the front of the book.
If anyone would like a translation of the legend then pm me and I can provide the legend in English language.
I have attached a snapshot of the cover of the book so you know what to look for when in country.
Hope this helps,