I'm learning Hindi as a second language. As a learner, I found this to be an enjoyable read and would say it is at about the intermediate level. I read an article about this book and always wanted to try reading it in the original language. I have had some difficulty finding reading material at the right level. I have read the Harry Potter books in Hindi translation (a great first step into reading longer works in Hindi) and found this to be near the same level of difficulty. Some slang, but not too bad to follow. Before this I tried an Agatha Christie novel in translation, but found this one easier. Probably because it is more action and dialogue-driven - and honestly, a lot more fun to read anyway.
I picked this particular book because there is also an English translation available. I thought I would read it along with the original. Thus, this would be a great way to get into ore of this author's works - in fact, I have a few others on my shelf. However, I ditched the English translation pretty quick - it was not literal enough to be very useful. That is, a lot of the difficult words or passages were not translated anyway (just left-out), so I stopped using it. It is not a bad translation, just surprisingly not very useful to read side-by-side with the original.