A Trail Through Time by Jodi Taylor

This is the fourth story in the St Mary's Chronicles and has to advance the story from the catastrophic ending of book 3.  I'm not sure if I'm 100% for the way they did it, but that's how it starts and this one is the most action packed of all the books.

The Time Police are after Max and she and Leon try to keep one step ahead of them as they jump across time, sometimes they are only half a step ahead.

The main adventure is set in Pompeii and, as usual, nothing goes to plan.  This all builds up to the final fight, which may well be the final fight of all.

I really enjoy this series.  As the books go on the characters develop.  Max is not the young newcomer from the first book anymore. The supporting cast are well rounded and bring more to the story with their St Mary's quirks.

http://www.amazon.co.uk/dp/B00L3K1AU0/?tag=jookuf-21