As with other series of children's of books from Harry Potter downwards, the first is often the best. The author has spent months if not years making the characters and plot believable, unlike sequels where - in the Darcy Bussell, Rainbow Magic, Too Ghoul for School series amongst others - subsequent books too often become formulaic as the publisher demands that the same book gets rewritten again and again to make money for all concerned.
This is the one where we first get to meet Antonia and her family, her friend Sophie, the nasty schoolfellow Draco sorry Lauryn and get introduced to the concept of silver dolphins and what they do. (Mostly get summoned at antisocial and inconvenient times of the day by dolphins to shift bits of debris from the sea bed). The main character - Antonia Lee - is believable and less nauseatingly sweet than the heroines of other books for prepubescent girls. In this book, as well as pollution she has to battle hazards ranging from bullying to a best friend who is less hung up on sea life than she is. After a falling out they have a reconciliation at the end but - understandably, given the books are about dolphins - the best friend takes an increasingly minor role in subsequent books.
Anyway, this is pretty well written, has a good environmental message and is recommended for girls of 6-11, although the younger end will probably need it reading to them. Some of the later books in the series are a bit repetitive and formulaic but if you've read the first three or four you're probably happy enough with what you're getting.