I have vols 1-4. When Baensch wrote the first volume he included the fish that were in the hobby then, and also added a section on plants, and one on diseases. The fish are most of the common ones, including a lot of tetras, some catfish, the common livebearers, cichlids etc. Vol. 2 has more fish and more plants. One may note that goodeids seem more or less a new addition for this volume. Vol. 3 yet more fish and plants. Vol. 4 more fish. One may note that a lot of livebearers and catfish come into the hobby late- that section grows in later volumes. But the common plecs and the platies and guppies were there in volume one. I would start with vol. 1, unless you are planning to specialise on goodeids.