First of all, let me say that Eoin Colfer's W.A.R.P.: The Reluctant Assassin is a great read. The author shows his usual mastery of plot and character and evokes general late Victorian London in intriguing, lifelike detail.
I say "general" because he chose to assign the past part of the story an exact year -- 1898 -- a date which doesn't line up with some of the elements. This isn't a minor gaffe, like another book I read awhile back that describes people wearing the latest miracle dye color, mauve, half a decade before mauve was first synthesized.
In this book, an invention stolen from the future and prematurely introduced into the past, the "Farspeak," has more than passing significance. In real history, while the telephone wasn't common in 1898, it had been patented in 1876. Even more egregious, the Old Nichol rookery, an especially bad part of the Bethnal Green slum, is a location crucial to the plot. Bethnal Green was razed in an urban renewal project in 1891, and rebuilt to sturdier and cleaner standards over the next decade.
So why did the author insist on 1898? I suspect it was to realistically accommodate a cameo appearance by Jack the Ripper in the backstory.
Still, the book is definitely worth reading.