I've read volumes 1 and 2. Volume 3 was a little boring, so I didn't finish it. It's mainly about Single-celled Organisms, Sponges, Marine worms, Crustaceans, and other micro organisms in the marine aquarium including, Comb jellies, Entoprocts, Horseshoe worms, Moss Animals, Lamp Shells, Water Bears, Sea Spiders, and Horseshoe crabs. I think it's a good reference for when you want to know something about one of those animals. But a straight read through was difficult. The first 2 in the series are about the corals we keep, so it's easier to read though them. I haven't seen book 4 yet. What's that one about John?
I would agree with John's opinion on the series. I was just a beginner when I tried the read the first book. I also had Sprungs first book, The Reef Aquarium at the time. I found that it was an easier read for me. As my knowledge and interest grew in this hobby, I reread Fossa's and Nilsen's first book and found it much easier.
So, when customers ask for books at the store, I recommend The Reef Aquarium to the beginners and the Fossa/Nelsen books to the more advanced hobbyist.