There are two dominant groups of Iron Maiden fans. The first insists that nothing they've done since Seventh Son of a Seventh Son is worth listening to, and the second insists that everything (or nearly everything) that the band releases has its own spark of genius. I've tended towards the latter approach, yet I was a little concerned when the announcement came that this, the 16th Iron Maiden full-length, would be the band's first double-album. Past post-reunion albums from Bruce Dickinson and the boys have been solid to excellent if often bloated and self-indulgent, which isn't a great setting for a double-album… And as with all double-albums, The Book of Souls is self-indulgent, and as with many, it's bloated!

It's still worth your time, however, because as much as we might wish for a blunt return to past glories, Iron Maiden have become decent hands at writing lengthy, prog-tinged heavy metal songs. You really have to be patient with The Book of Souls, which perhaps goes without saying, but some of the better moments only come out with familiarity. The only track I loved from the first listen was opener If Eternity Should Fail, which opens with fuzzy synths and an echoing Bruce, who sounds terrific for a man who was recording with two tumours in his mouth! Building to a typically grandiose chorus, the song repeats the chorus between enjoyably proggy instrumental dalliances that work familiar Maiden riff tropes in well. The song is let down a little by a too-weird spoken word section at the end, and some pointless acoustic strums; not the last thing I'd have cut from the album. Yet the band sound as good as ever, if not exactly champing at the bit – for perhaps not the first or last time, Iron Maiden are sounding their age.

This is especially clear on the following track, first single Speed of Light, where apart from an opening yowl from Bruce, he sounds strained at moments and isn't helped by the song, which while likeable enough and sure to appear in live performances is forgettable against material even on the same album, and woefully average compared to the likes of The Wicker Man. For all the talk of how progressive the band are, they have become pretty predictable songwriters in the last fifteen years, and there's little here that couldn't have come from the sessions for A Matter of Life and Death or The Final Frontier; The Great Unknown one example of this, moving from (admittedly superb) lead guitars to quieter, atmospheric melody exactly when you expect it to. It adds nothing to the tracklisting and I'd be amazed if many called it anything other than outright filler. As a lot of the material here, it lacks the personality that usually shines from Maiden songs, even on The Final Frontier.

That you come away from The Books of Souls with clear favourites after even this is proof of Maiden's lasting power. The Red and The Black is classic Steve Harris in epic mode, with Bruce spitting out lyrics in the verses and 'whoaa'ing in the chorus – as becomes increasingly plain throughout, however, the real stars of the show are guitarists Adrian Smith, Dave Murray, and Janick Gers, who make their instruments sing and provide the emotional kick that we know and love from Maiden. Whether noodling, riffing, or soloing, they drive the song onwards, providing a truly epic impact and proving the triple-guitarist gimmick is anything but. The enjoyably rocking When The River Runs Deep is further proof of this, and as the first track on the tracklisting that doesn't feel overlong, has to be recommended. And ending the first disc with the title track was a wise move, as the mid-paced ten-minuter has an almost Eastern vibe akin to Powerslave, updated well with their modern sound but still making space for a burst of very headbangable speed metal riffing. Fair play to Bruce, too, for nailing each and every word like a man half his age!

The second half of the double album is notably weaker, and the more listens I give to The Book of Souls the less this seems like mere fatigue. Death or Glory is slower than a track about dogfighting should be, and suffers from some downright odd lyrics – 'climb like a monkey'? Tears of a Clown, the Robin Williams-themed piece, opens well with some atypical riffing but again is let down by the goofy lyrics, and doesn't really go anywhere. Shadows of the Valley, however, is another good epic, more upbeat than the surrounding tracks if not quite up to the epics on the first disc and with an intro that seems very similar to that of Wasted Years, and The Man of Sorrows has a slightly psychedelic air that works well with the proggy riffs, sounding like something from a Dickinson solo album – ironic given a track with a very similar name appeared on 1997's Accident of Birth. The best track on the second album, however, is closing eighteen-minuter Empire of the Clouds, surpassing Rime of the Ancient Mariner as the longest track in the Maiden catalogue. It's not as good as that classic, but is surprisingly great for a track driven primarily by Bruce on piano. Written about the 1930 R101 airship crash, the track is the proggiest here, going so far as featuring sections played in Morse Code! It's a microcosm of the double-album, moving from section to section well and featuring a great performance from Bruce but suffering from its length and quite capable of being cut by a few minutes.

The Book of Souls, as a whole, is very much like the artwork to Dance of Death to me – a first draft accepted when further revisions would have made something much better. Iron Maiden, like many other bands of this longevity, need an editor, and one would have helped reform The Book of Souls from a ninety-minute bloated monster into a sixty or even seventy-minute masterpiece. Even generally forgiving Maiden fans will find moments, if not full songs, that they would have cut, but it's a tribute to this band that I never felt like my time was actually being wasted by The Book of Souls. It's also good that Iron Maiden are still challenging themselves, and their listeners, even as they approach what must be the twilight of their career. The Book of Souls is a challenge, but it's one worth undertaking despite its flaws.