The first chapter (on the internet and internet services) goes into a little too much detail about such things as ftp sessions and the distinction between clients and servers. This will scare some readers unnecessarily (I can't see the average Macintosh user wanting to know what a Unix ftp session looks like!), but starting with chapter two things become much more user friendly, with a slow, step by step treatment of the material, complete with illustrative snapshots of Mosaic windows. Chapters two through four cover the basics — getting Mosaic up and running, the World Wide Web and non-Web resources like gophers, WAIS and News. Chapters five through seven cover extras — customization of Mosaic, using multimedia with Mosaic and creation of your own HTML documents. The final chapter has information about the W3O project and sources of information on the WWW, the appendices have some reference material, and a glossary and an index finish off the book. This covers everything ordinary Mosaic users could want, and is sensibly structured. All the material is clearly laid out and explained, and The Mosaic Handbook is up to the high standards one expects from O'Reilly.
I haven't seen the Microsoft Windows version, but I assume it is very similar, since the X and Macintosh versions are pretty much the same, with the differences concentrated in the chapters on multimedia and customization. (The X book is so much longer partly because these are more complicated with X and partly because the window snapshots take up more room). I have two minor gripes — there's no Linux binary on the CDROM, and the X Windows window snapshots are barely readable (I think some verisimilitude should have been traded for legibility) — but otherwise have no qualms at all about recommending The Mosaic Handbook.