In my opinion these two languages are very close. There are some differences in grammar, but I used to have almost no trouble reading technical literature in Bulgarian. I think I could read fiction as well if I spent let's say a month on learning a little more of the grammar and some vocabulary. I have a Bulgarian dicitionary somewhere, which lists so called false friends of a translator, i.e. the words that have the same spelling in both languages but completely different meaning... Having said all that, understanding spoken Bulgarian is a different topic altogether. I can't understand a word Although to tell you the truth I heard it spoken only on a few occasions...