To be honest, I still do not understand the reason behind the artificial spellbook cap using the highest class spell level. If you have enough resources to raise the spellbook and they are directly applied, the cap does not make any sense.
The spellbook should be raised as high as possible given the resources available. If the character is not 50 yet, the spellbook is “level-adjusted” automatically as the equipment is.
Why is a class spell level cap pointless?
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if the resources are enough to raise the spellbook above the class spell level cap, the unused resources should be compensated. Once they are compensated and the player is in 2.5, the first thing will be to upgrade the spellbook to the maximum possible level given those resources. This is now possible, as there is no class spell level to cap the spellbook (no more spell levels in 2.5).
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if the resources are enough to raise the spellbook above the class spell level cap and the resources are not refunded, then the player loses resources, which is a really bad thing.
Example: If for some reason, I like the level 35 value of class spells, but raise several to legendary (for the sake of the argument I decide not to increase any to lvl 45). Now the spellbook cap will be set to lvl 35, but the resources from multiple legendary class spells will be applied to the spellbook, which should raise the level to > lvl 35 (as the resources also include several glyphs).