Hakulinen L. The Structure and Development of the Finnish Language

Indiana University, 1961. - 383 p.

This book discusses both the grammar and the historical development of the Finnish language.
Despite being somewhat outdated, this book contains numerous gems that are otherwise hard to find in other more mode works: syllable structure, plural/singular special uses and other grammatical issues.
Of special interest is the extensive treatment of the Finnish lexicon, including questions of word origin (native vs. borrowed) and word formation.