In addition to a spelling notebook, Aidan also has a grammer notebook. Montessori uses symbols to represent word types for a version of sentence diagramming. (See an example, here.)
This year he is keeping lists of words in a notebook as a word bank for sentence writing. We just stapled 5 sheets of white paper together and he decorated the cover with the symbols. On this inside cover he started a master list of the types of words we have covered so far (article, noun, verb, adjective, & adverb), with room to add new ones this year. Then each page gets a symbol and a word type at the top and he can add words to create his word bank. This week I had him add his spelling words (picture above), but he can also add interesting words he comes across in his literature reading.