For the developer approaching the UML for the first time, the user guide is best read linearly. You should pay particular attention to Chapter 2, which presents a conceptual model of the UML. All chapters are structured so that each builds upon the content of the previous one, thus forming a linear progression.
For the experienced developer seeking answers to common modeling problems using the UML, this book can be read in any order. You should pay particular attention to the common modeling problems presented in each chapter.