This book is a manual of the Finnish language, describing its features and difficulties in addition to the basic grammar, word derivation information, spelling and style guidance, etc. This is not a textbook but a reference. The idea is that you can use the book to check specific issues in Finnish that are important to you in some occasion. For example, a reader with limited or maybe no previous knowledge about Finnish can consult this book to find information about a particular phenomenon or language form in Finnish, about the way to write some notation in Finnish, or about some specific difficulty in translating into Finnish.
However, the book can also be read systematically to get a more thorough understanding of Finnish and to find new interesting possibilities or pitfalls in the language. For this, you probably need to know Finnish well enough to read simple newspaper text or to use Finnish in simple everyday conversation.
In principle, no previous knowledge is assumed, but in practice, most people would find this book hard to read if they do not know Finnish at all—unless they are linguists or polyglots. The problem would be lack of motivation and general framework. For example, due to the systematic approach, the book has a long discussion of variation of word stems and suffixes before it deals with specific suffixes and their meanings.
The links in this book mostly refer to pages in English. However, there are some links to material in Finnish, for readers who wish to read more thorough descriptions of things discussed here. Such material may require fairly good understanding of Finnish. You might wish to try to use some automatic translation software to get an idea of the content, though. Such links are marked in a special way; example: Iso suomen kielioppi (ISK). ISK is a large descriptive grammar of Finnish; the printed version has about 2,000 pages. For features of Finnish, ISK is the basic reference in general; therefore, this book does not usually link to ISK but to specialized material elsewhere.