A shelf or two full of general gardening books for reference are great to have. Unless you have been doing gardening for a living for many years it is unlikely that you will have all the knowledge to hand on any given day. It is also unlikely that you will only tend to a particular type of plants and they all need different care, soil, nutrients, sun, water, etc.
If you grow trees, they will have different preferences, similarly different types of shrubs and flower plants. You may be interested in growing vegetables and herbs in the kitchen garden, or more specifically, doing organic gardening. Whatever type of garden you have, a good set of books is always useful to refer to.
