Here's the breakdown:
1. The Original Title: These are the titles Meg Cabot used when she first wrote the books. They were aimed at young adult audiences and often had a more informal, playful tone.
- *The Princess Diaries*
- *The Princess in Training*
- *The Princess Proposes*
- *The Princess Keeps Her Promise*
- *The Princess Gets Married*
2. The "Deluxe" Edition Titles: When the books were re-released in a more polished, "deluxe" format, they got slightly more sophisticated titles aimed at a slightly older audience.
- *The Princess Diaries* (same as original)
- *The Princess in Training* (same as original)
- *The Royal Wedding*
- *Princess Mia*
- *Mia in Love*
3. The Film Adaptation Titles: The film adaptations of the first two books (and the subsequent sequels) used shortened, more concise titles, often mirroring the original titles.
- *The Princess Diaries* (same as original)
- *The Princess Diaries 2: Royal Engagement* (based on "The Princess Proposes")
- *The Princess Diaries 3: Royal Wedding* (based on "The Princess Gets Married")
In summary: The different titles reflect different stages of the series' publication history, as well as attempts to appeal to specific age groups and audiences.