Does the Book of Exalted Deeds allow level 10 spell slots?

Question

The artifact Book of Exalted Deeds, DMG p. 222, has the following attribute:

Enlightened Magic. Once you've read and studied the book, any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher.

Therefore, if a level 17+ cleric were to read this book and subsequently cast a spell that scales with slot level, such as Cure Wounds - would he be able to cast the spell at "10th level"?

Cure Wounds:

[...] When you cast this spell using a spell slot of 2nd level or higher, the healing increases by 1d8 for each slot level above 1st.

9th-level slot: 1d8 + 8*1d8 = 9d8
9th-level slot with the Book of Exalted Deeds: 1d8 + 9d8 = 10d8?

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Clarification: stating that there currently is no explicit mention of 10th-level slots or spells is not helpful, I'm fully aware of that - it's the whole point of this question.

Therefore, unless you can provide a reference to an official source that explicitly disallows 10th-level spell slots, please refrain from answering. Note that, while they're generally not considered official, a quote from any designer (not just Crawford), will suffice.

Answer 1

Yes

That is what it says it does, so that's what it does. There're no serious problems with this, either, yet; the Book of Exalted Deeds is an artifact level magic item and (as your example shows) most spells just work a little better with this effect. The only potential issue comes from the ability for a spell to be written needing slots higher than 9th not to exist (since the developers will continue to expect 10th level slots to be impossible, probably), but no such content has yet been created.

Answer 2

No. You can't turn up the volume to 11.

Spell Levels are currently limited to Level 9. There is no class that has a 10th level spell slot and there are no rules that suggest there is a 10th level spell slot. Neither are there current 10th level spells that could use a 10th level spell slot.

The PHB (page 201) clearly states:

Every spell has a level from 0 to 9.

and (emphasis mine from @MichaelPolitowski)

When a character casts a spell, he or she expends a slot of that spell’s level or higher, effectively “filling” a slot with the spell. You can think of a spell slot as a groove of a certain size—small for a 1st-level slot, larger for a spell of higher level. A 1st-level spell fits into a slot of any size, but a 9th-level spell fits only in a 9th-level slot.

This clearly states that there is no 10th level (currently) spell that you can cast. While it doesn't explicitly state there is no 10th level slot, the implication is that without 10th level spells there aren't 10th level slots - otherwise there would be upscaling options allowing you to utilize that.

One can reasonably extrapolate how it'd work, but the current rules do not include 10th level and there are no other examples list going to a 10th level (such as examples that list surpassing 20 for an Ability Score.)

The following phrase from the Book description does not create a new spell slot level, it simply upcasts to one level higher. The current cap of a 9th level spell remains:

any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher.

Unlike the rules around Ability score raising with the Tomes/Manuals, this book does not state that it increases spell levels beyond the current cap of 9th.

A free upcast

What this book does do is give you a free upcast without having to use a higher level slot. That's still a pretty great benefit without having to create a 10th level spell slot that just doesn't exist in the current rules.

The stated benefits are all still applicable and do not require generating an unlisted option (10th level spell slot) in order to be useful and/or work.

Interactions to consider and the Slippery Slope

If you do allow this in your games, be aware that this changes Counterspells and Dispel Magics. A simple fix, but just something to be aware of.

And for a warning, if this allows 10th level, then why couldn't someone create a 10th level scroll? Or an 11th? Once you've confirmed the existence of 10th level, then that level now exists and must be accounted for.