Have you read "The Private Life of Ghengis Khan", "The Meaning of Liff", "A Christmas Fairly Story", "Last Change to See" or "The Long, Dark, Tea Time of the Soul"? If not, then feel free to read "Dirk Gently", as there will remain DA works you haven't read.

I've not read "Ghengis Khan" or "Christmas Fairly Story" myself. And I'm not even counting unpublished/unfinished works which may have been adapted/completed by third parties ("Doctor Who and Shada", the unofficial novelization out of New Zealand?)

What is your opinion (as in: is it part of the Trilogy) of "Young Zaphod Plays It Safe"?