I haven't read many books dealing with pure psychology (just papers or books on specific areas of psych), though in my first year class we use this book as our textbook:
Psychology An International Perspective
It's very easy to understand and, from what my lecturers have said, it would cover everything that you'd want for the level of knowledge you're after. It's a big book so I doubt you'd need more. And in this and any area of science, you're probably better off looking to recent research or review papers to fill in the gaps left by a book (or to keep up with corrections in your area of interest (since it's impossible to keep up with everything))
As for sadavis' comment regarding reasons for gathering all this knowledge, the more you know the easier it becomes to create links between new knowledge and existing knowledge, and you'll be able to create
more links, making
all your knowledge more accessible, not necessarily just the particular field you're working in.
For example, if you know a lot about computers it's possible to come up with analogies that help you understand more when you're learning about neuroscience. And vice-versa. (no the brain isn't a computer, nor is the computer a brain, but the analogy can be useful as long as you're aware of where the analogy breaks down).
An example which is clearly applicable to many fields is networking theory. Computers, biology (including neuroscience), social psychology... All of them involve networks at some level.
Finally, the association of differing concepts to create new ones is a big part of creativity (arguably
all of creativity).
(not that I need to convince you Netopalis, but if anyone else doubts why this is a good idea, hopefully they can put their doubts to rest)