Even people who speak Finnish well, perhaps as their native language, may find this book interesting. An analysis of a language feature may give you a deeper understanding of something that you use fluently at the practical level. Moreover, such understanding helps a native speaker of Finnish to avoid language errors when speaking a foreign language: we all tend to use features of our native language in other languages, too. This book also describes features that are not in widespread use in modern language but help you to understand older texts—and maybe to enrich your own texts with carefully used “old Finnish”.
In particular, this book documents phenomena that are common in Finnish but generally not described in textbooks or grammars. For example, actual pronunciation of foreign names in Finnish often greatly differs from the instructions given in encyclopedias and other sources.
People learning Finnish at an early phase may find descriptions in this book useful for orientation, for explaining background, and especially for dealing with complicated phenomena, such as word inflection. Language learning is both cumulative (you just add new words, phrases, etc., to the repertoire you have) and perceptional (you learn completely new concepts and ideas). This book mainly helps with the latter, though as an aside, you will learn new words as well.
People who know Finnish well can read this book systematically to improve their command of the language, written and spoken. This is useful especially to people who have mostly learned the language by the natural method. This book can be also used as a reference book when you are uncertain about some features of the language.
Translators, interpreters, and localizers can use this book as a reference to Finnish grammar, orthography, and to some extent style. This book discusses both normative rules of Finnish and common usage; the latter may be relevant e.g. when translating informal material or dialogues.