Packt Publishing's ever growing Drupal bookshelf expanded with Drupal 6 Performance Tips in February. The book rightly earned its title from being some tips without trying to pin down a comprehensive guide. Maybe even better titling would have been to attach a "for beginners" to the end. Contents of the book really cater for beginners with little coverage of topics outside Drupal's (and some major contributed modules) own internal solutions to performance problems. Its subtitle hints that it provides best practices which I'd respectfully dispute.
What's entirely surprising is that the 220 page volume starts off with 50 pages on upgrading a Drupal 5 site to Drupal 6. While the most current performance tweaks are usually documented for latest versions of Drupal, assuming the reader must have started off with an old Drupal site might not be the best way to set the tone for the book. However, the author comes back to upgrading modules regularly at a later stage in the book, which I've valued. It clearly underlines the need to come back and update your environment once in a while. (That module upgrade explanation is at an absolutely unexpected point in the book, which makes it even better positioned).