For my regard, The Book of Souls aged quite well. My main concern about it was and still is the poor production, which I find outrageous for a band like Maiden.
If Eternity Should Fail works well as an opener and it's an overall good song. I can't wait to hear Roy Z's version.
The first single, Speed of Light, as per tradition, is the worst track out of the album.
The Great Unknown is interesting, but Bruce is clearly out of his range in the pre-chorus. He sings so high he can't articulate words. Honestly they should have tuned it a half, maybe even a whole step down.
The Red and the Black is basically the loser little sister of Rime of the Ancient Mariner ft. Uriah Heep's I Hear Voices in its first half. Then Harris stops to cover himself and his idols and brings in a good transition chorus ("The reeeed aaaand theeee blaaaack" etc.). Cool riffs in the instrumental section, maybe some forcing here and there. Also I wish they had left the coda as a pure instrumental, as I don't like the "who-oh" reprise.
When the River Runs Deep is a good up-tempo filler. Basically The Alchemist with three solos.
The title track in itself is the masterpiece of the album, and if they hadn't written The Legacy, Dance of Death and Sign of the Cross, I would rank it as their best track since 1988, approximately. The pre-chorus riff is The Edge of Darkness' main riff and the main fast riff is basically Losfer Words.
Death or Glory is a good, old style song.
Shadows of the Valley starts with a lame and useless plagiarism from Wasted Years and Isle of Avalon which the "Sea of Madness" line can't really turn into a divertissement, then starts The Fallen Angel merged with Where Eagles Dare, then again they made a cool medley with The Reincarnation of Benjamin Br-- wait, that's the chorus. Janick's instrumental melodies and the final "Into the valley of death" chorus are the best sections of the song.
Tears of a Clown is, quite frankly, another filler.
The Man of Sorrows is the second and last good song of CD2. A bit repetitive in the first section, but it gets better as it proceeds. Interesting structure, cool Blackmore-ish solo by Janick, interesting outro solo by Dave.
Empire of the Clouds ranks barely 6/10. It's an interesting experiment and the lyrics are cool, but it lasts too long and, above all, features the umpteenth self plagiarism of the record.
It's not a bad record, it's better than The Final Frontier, and it's basically what I would expect from Iron Maiden in 2015. I would rank it a 6.5/10 (-0.25/0.5 due to the production).