We found this a very useful & to the point guide. Advise you to read it before going and then keep it in your bag whilst out & about in Havana. we would never have known about the Princess Diana Garden but for this book and it is so beautiful. Many of the smaller museums are lovely & worth visiting but doubt we'd have found them if we hadn't read this book.
I haven't been in Havana yet so I can compare with reality but it seems to be written in short and efficient way. It has good tips to know as Cuba is still communistic country so it's essential to know some stuff before you go.