For the beginner: For an all-around guide to basic water chemistry and cycling, aquarium husbandry, and livestock care:
The Manual of Fish Health by Dr. Andrews
For a more in-depth and updated guide to fish disease and health:
Fish Disease: Diagnosis and Treatment by Edward J. Noga
For an excellent set of diagnostic charts for identifying illness in fish:
Handbook of Fish Diseases by Dieter Untergasser
For identification of fishes:
Aquarium Atlas by Hans A. Baensch
And, because I am also interested in Aquarium design and plants and you may also beA:
For a basic introductory guide to plant identification/selection, water chemistry requirements, fertilizing, Co2 et al...
Sunken Gardens by Karen A. Randall
Pure inspiration in aquarium design:
Nature Aquarium World by Takashi Amano
An indepth guide to all things aquatic plants; with colored pictures of aquarium plants, descriptions, native conditions and requirements in aquarium:
Aquarium Plants by Christel Kasselman
Then, once you are proficient in this- go to the following. It is not a starting point- but, an end-point; when you are proficient in the above.
Planted Aquarium by Diana Walstadt