In the last novel, Phyllis Wong And The Return Of The Conjuror, she discovered that Wallace Wong was still alive and well and travelling through time(or, rather, Time), in a process he calls Transiting - and that she, too, had the ability to do this. There's no TARDIS. If you have the ability you can do it, with the help of some stairs and an object from the Time you want to visit, and you can take a guest with you - in Phyllis's case, this is her friend Clement, a boy who loves over-the-top disguises and playing zombie fighter games online. If you don't have the ability, you can run up and down the stairs all you like and you'll just get tired. That novel was about a lost play by Shakespeare and some suspiciously new but absolutely authentic First Folios being auctioned off in the present day. There was some time travel involved.
This novel involves more time travel, a Paris theatre in 1931, an evil ventriloquist and Myrddyn Emrys, aka Merlin. Wallace Wong does make an appearance but leaves the story early, hoping that his great grand-daughter will find Merlin, not only the greatest magician of all time, but the inventor of the TimePocket used by Wallace and Phyllis. As the story continues, it becomes vital that she does find Merlin or the world might just come to an end, not with a bang but with the Great Whimpering...
In some ways this series is very different from Geoffrey McSkimming's Cairo Jim Chronicles, in which an Indiana Jones-like archaeologist had adventures in various parts of the world, with his companions, a Shakespeare-quoting macaw and a telepathic camel who enjoyed reading western novels. There was even a kind of Marcus Brody in those novels. The heroes of this series are a young girl and her friend and the time is clearly now, with the Internet and mobile phones, while you never could tell when any individual story was set in Cairo Jim; a couple of them had mobile phones while in another of them a character remembers something that happened in 1910.
But there is the same over-the-top whimsy, the same humour. Wallace Wong keeps making bizarre comparisons and, when Phyllis doesn't get them, exclaiming, "Oh, I know what I am meaning!" And there is also a message; in Cairo Jim, the gentle message tended to be about countries that appropriated the cultural heritage of other countries, through their museums. In this one, interestingly, a message of sorts comes from the lips of the villain! He's right, but also wrong. Read it and find out what it is.
It's probably better if you have read at least one of the two earlier books, but it isn't necessary. I haven't read the first one.
Recommended for good readers from about ten upwards.