It's a bit dated but well worth the read:
Listening to Whales : What the Orcas Have Taught Us by Alexandra Morton (2002).
However, combined with Gabriela Cowperthwaite's poignant documentary Blackfish (2013, available on DVD, 83 min), it's a strong indictment of the entertainment industry which exploits sentient and intelligent beings.
If that doesn't turn you into an 'animal activist', nothing will. (A good start would be to scuttle SeaWorld exhibits wherever they are and by whatever methods you feel comfortable with.)
