Iso suomen kielioppi. A voluminous collection of articles on Finnish grammar (1698 pages in printed form). Also freely available as a web version, Verkko-ISK, VISK. It is a descriptive (not normative) grammar, written in a more or less academic style.
Suomen kielen rakenne ja kehitys. A monograph on the history and structure of the Finnish language. Last modified edition published in 1979.
Kielikello. Initially a periodical published by the Institute for the Languages of Finland, from 1968. Since 2024, new articles are added frequently without organizing them as issues. It is freely available at Kielikello.fi. Includes both practical guidance to Finnish usage and articles on the language.
Korp, at kielipankki.fi/korp, is a search system for texts in Finnish, containing 1,249 corpora, with close to 5 billion word occurrences. Though it has user interface in English, too, its use requires better than elementary understanding of Finnish, some patience, and some experience with the system. In searching for occurrences of a rare word in contexts, it can be valuable. However, it only shows short fragments of texts, mostly with no access to the corpora as such.
Nykysuomen sanalista is a word list with about 100,000 records, with inflection information. It can be used as test material for linguistic software, for example.
Verbix.com shows all inflected forms of a verb.
Oikofix, at oikofix.com, is a free online spelling checker and word analyzer. The analysis shows the morphological structure of a word using Finnish grammar terms.
HFST is a different word analyzer. Its analysis uses abbreviations of English terms for Finnish grammar construct.
A sample analysis by HFST as well as by Oikofix is shown in section Analyzing a word.