I highly recommend Kant by S. (Stephan) Körner. It is a minor classic, I am not sure if it is still in print & there have been many versions of it. There are quite a few second-hand copies around. I found a Pelican copy of it a few years ago which was older than I am. I paid 99p (UK currency) for it but according to the cover when it was printed it was 3/6! (17 & a half pence in decimal).
It is probably the best introduction to Kant's often puzzling concepts I have ever read. I have read it several times.