We recently started re–using LaTeX for large documents — professional computing books. Years ago (1987) I had personally used a LaTeX 2.09 custom class (although it wasn’t called a class file then) for a book I was writing. The class file was written by a colleague, because I found it impossible to understand the low–level TeX mechanisms needed then.In 2006 I wanted to use this class for a new book of my own, and also as the basis for all our camera–ready books submitted by other authors.I had the choice of converting it to a new .cls file, which much of the documentation suggests is the thing to do, or writing “add–on” .sty file for the standard book.cls. We decided to try the .sty approach, as there were several packages that seemed to do most of what we wanted. We found that the .sty approach was straightforward, and much easier than expected. The resulting style is much shorter, easier to understand and maintain, and is much more flexible than our old one — we can use most standard packages to add extra features with no effort, because we still provide all the hooks that add–on packages rely on.