Consider the fllowing scenario: a level 20 Wizard duplicates his spellbook and hides one copy. The he casts True Polymorph on himself, becoming a Planetar Angel (the original copy of the spellbook disappears). Finally, he recovers the copy of the spellbook that he hid.
1) Is the Angel able to use the spellbook and cast the spells he learnt as a Wizard?
2) If so, does he have the spell slots indicated in the Planetar Angel sheet?
3) What would change if he used Shapechange instead of True Polymorph?