If you have Martyr there too, then Altan has less added value indeed... but still, in general I'd prioritize books for reducing cooldowns and increasing chances to apply debuffs, over books that only add some extra damage. Look also at other content than CB where you use those champs. But okay, if you really don't need Altan to put more increase def / decrease attack, then maybe it makes sense in your specific situation to go for maximizing damage on your Septimus first.