To discover the similarities and differences between primary and secondary computer manuals, and to account for the popularity of the secondary texts, two best-selling books for word processing and spreadsheet programs are compared to documentation supplied by the manufacturer. A heuristic for analyzing software documentation based on cognitive and rhetorical principles is developed and applied to the corporate documentation for (WordPerfect 5.0) in contrast to Stewart's Using WordPerfect 5 from Que, and the corporate documentation from 'Lotus 1-2-3' in contrast to Gilbert and Williams's 'The ABC's of 1-2-3 from Sybex.' It is shown that the trade texts from Que and Sybex contain more conceptual background information than the corporate documentation and differ in their rhetorical stance: the writers provide a richer context by giving more examples for applying the software; the writers provide global and structural frameworks; the writers use persuasive marketing techniques to ease the reader's anxieties and remind them of the software's benefits; and the writers identify themselves.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>