Today I came across a wonderful 'grammar' resource in my stash from mom; Dear Peter Rabbit by Alma Flor Ada. Andrew learns grammar mostly by osmosis during reading. We have (rarely) done formal language arts and when I do it doesn't really stick. We have also had pen pals from time to time and that has helped a bit but mostly Andrew absorbs through reading. This wonderful gem of a book is a series of 'letters' written to and from various fairytale characters such as Peter Rabbit of course but also Baby Bear, Goldilocks, Pig One, Wolfy etc... It has proper letter formation throughout the book. We have not read yet but plan to add it to the book list this summer.
note: For a long definition of a living book see here. For a shorter definition look here. The following excerpt is taken from the short version.
Living books, often called "classics," are the kind of books that joyfully enliven the imagination of a child. They are written by individuals -- not committees -- and display imagination, originality, and the "human touch." Living books do not talk down to a child's level or omit odd and interesting vocabulary. Children take to living books more than textbooks for these reasons; because such books are not crammed with facts and information at the expense of human emotion.