Taxi company in Sofia, providing also cabs to Plovdiv (185 BGN). It can be rented per hour (min 3 hours/30 BGN per hour) to visit Sofia sights. There is an office at the airport in Sofia, in Arrivals area.
How to read in Bulgarian - most of the signs in Sofia and Plovdiv are transcribed in Latin letters, but it might still be useful to be able to read signs in Cyrillic letters - it is very easy since Bulgarian language is phonetic