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?
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.
Best Answer
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.