I bought this book in 1996 when I entered graduate school and have utilised it as a reference while writing all my research papers as well as my dissertation. I also have "The LaTeX Companion" by Goossens et al., but this book is much more useful to me. At about 170 pages, it crams everything you need into a very short amount of space, including info on fonts, tables, math, formatting, etc. The chapter on large documents was invaluable to me when I was preparing my dissertation. Furthermore, the last chapter has a number of cool macros that have come in pretty handy.
Note that this is not a beginner's book on LaTeX, nor does it exhaustively show all the extremely cool bells and whistles that LaTeX can do; this book serves as a concise reference and list of hints and tips for using the key markups. When I write a paper, I like to concentrate on the actual content I'm writing about rather than how to format the text. However, when I do need to format text just right, I turn to this book, and it has never let me down.