The word mittarion appeared in tengwar form on what seems to be a page of art for an (unpublished) book cover created by Tolkien, within the untranslated phrase parma mittarion (TMME/192). The general consensus is that this phrase probably means something like “*book of enterings”, where the word mittarion is genitive plural form of a noun mittare or mittar “entering” based on the verb mitta- “to insert, enter”. I think “entrance” is another plausible translation.