[antiform]

It certainly depends on what you're interested in. This is more of an encyclopedia of mathematics than a textbook. If you want an broad idea of many different fields of mathematics and the history behind them, presented in a well-written and relatively elementary style, this is the book for you. However, the problem with being relatively elementary is that most of the articles merely skim the surface of whatever subject they are describing.

If you want to actually be able to solve problems using math (the fun part, for most amateur mathematicians), a textbook would be a better bet. If you stopped math at business calculus and want a great introduction to the calculus that mathematicians use, I wholeheartedly recommend Michael Spivak's Calculus as a next step.