When I was in 7th grade I first purchased this book, and I have used the guidelines outlined in it ever since. There is quite a lot of valuable information packed into the book! There are sections on Basic Grammar and Writing Elements, the Art of Writing, Personal Writing (such as in a journal), Subject Writing (such as a news story), Creative writing (such as poetry), Thinking skills, etc. One of the most helpful parts of the book is the "Yellow Pages" section, which is basically a grammar tour de force that explains subordinate clauses and other potentially difficult elements of English Grammar. Perhaps the most useful part of this section is the "Using the Right Word" section. It has commonly misused words, such as "Effect" and "Affect." It has each word properly defined and used correctly in a sentence as an example. Finally at the end is a tables of weights and measures, a periodic table, a Morse code alphabet, a world map set, historical documents such as the Emancipation Proclamation, etc.
The principal of my junior high, who was also a friend and neighbor, gave me a copy of this book for my fifteenth birthday. Useful throughout high school and college, and I still pull out my dog-eared copy when I have questions about how to format a particular style of letter, have questions about grammar, or need some general inspiration.
I have the 1990 paperback 1st edition. I am curious to see how this staple of my writing life has evolved. I understand there is now a .pdf on compiling multimedia reports on the Write Source 2000 website. Any basic book on writing, thinking, and learning that incorporates modern technology in this way definitely belongs on my shelf.
Update: I bought more recent, updated versions for both of my sons. They go to this resource all the time.