A YA prequel to Mary Shelley's Frankenstein (which kind of feels weird in itself), This Dark Endeavour imagines a young Victor Frankenstein whose twin, Konrad (not in the original), falls dangerously ill with a mysterious blood disease (leukemia, maybe?). Victor, desperate to save him, embarks on a somewhat quixotic quest with his friends Henry Clerval and Elizabeth Lavenza for an Elixir of Life, enlisting the help of a dodgy and illegal alchemist to translate the ancient encrypted texts and mix up the ingredients they so painstakingly acquire in dangerous and reckless circumstances.
It's the kind of book whose existence only really makes sense in relation to the original classic: as a standalone, it's vaguely dissatisfying, a tale of failure more than of success; more a "making-of Frankenstein" than a story in its in own right. I guess it would work best as an introduction to Frankenstein; it sets up that novel very well, and there's enough adventure and strange Gothickry to make readers seek out more.
If there's one thing that did annoy me, it would be the dialogue, a strange and awkward mix between Victorian and modern registers which felt rather artificial at times (I can't help comparing it to the last book I read, Naomi Novik's Throne of Jade, which does brilliant and thoroughly period-specific dialogue). Otherwise, everything is pretty much unobjectionable, if unsurprising: Victor and Konrad's rivalry for Elizabeth's love adds an interesting dimension to what is in Shelley's novel a weirdly bloodless (not to mention semi-incestuous) relationship; Victor's interest in alchemy expands nicely on his experiences with it in the original novel; Oppel's continual focus on science over alchemy, on the rational and knowable over the transcendent or magical makes for an interesting commentary on Frankenstein, given the difference between what we know now, scientifically, and what we knew then.
This Dark Endeavour, then, is pretty good as a retelling/prequel, well-plotted if occasionally over-sanctimonious (the Frankensteins making Sunday dinner for their servants? Really?). Also, it has a cute cat in it (OK, lynx, but it's a tame one), so that pushes the rating up a bit. Twistily Gothic, inventive, faithful to its original: it's not bad.