Another short to accompany the main St Mary’s series. While it’s perfectly possibly to follow the series without reading the short stories, I would certainly recommend them as it provides you with some extra adventures and postpones the inevitable moment where you’ll have to return to the normal, non-time traveling world.
Expect everything you’ve come to expect from the series. Now set in Roman times with Caesar (the one that gave it the name) and Cleopatra, because when you install your mistress in your wife’s house, this of course is asking for problems.
The only remark I have is that I would not read it as #3.5. Without going into spoilers, book three ends on quite an important moment with quite the cliff-hanger, and book four opens there, so personally, I would recommend to read it before starting A Second Time, as it probably takes place somewhere during the events of that book.