Kirja is a typical example of a root which appear in several words.
Kirja means “a book”. The verb naturally is “kirjoittaa”, to write (note also “kirjata”, to book or to log). Kirjoitus means writing in general.
The “-ja” suffix is similar to English “-er”, thus kirjoittaa -> kirjailija = to write – > a writer (or an author). Kirjasto means a place with many books, a library (“a bookery”). Kirjallisuus means the literature (“bookness”).
Käsikirjoitus is a direct translation from latin, a manuscript.
Kirjain is “a letter” as a letter in alphabet, but Kirje is a kind of letter you send in post.
On the other hand, English has some uses for this word which Finnish doesn’t have. For example you “book” an apointment in English. Finnish has another word for that “varata”.
I booked an appointment for a doctor./
Varasin ajan lääkärille.