This makes it sounds like a wizard would have a really hard time to prepare a spell from another's spellbook. Is that the case?
That is correct.
As you quoted, each wizard's spellbook notation is unique, therefore you can't prepare spells from someone else's book.
You must first copy the spell into your own book (deciphering the notation in the process), after which you can prepare it as normal.
The ability to read and cast spells is not restricted by the new form (Planetar).
- To answer your second question first: Shapechange explicitly does not permit use of the new form's class levels or spell casting traits (the wizard would not accrue the innate spell casting feature of the Planetar) but the wizard
will retain the benefit of any features from your class, race or other
source, and can use them provided that your new form is physically
capable of doing so. (PHB p. 274-275)
The wizard can thus cast his spells. An angel/Planetar is able to cast a spell and to read. (It might be wise to hide an arcane focus with the spell book to avoid the problem of spell pouch/focus changing as well). The Planetar's spell casting without components is linked to its innate spell casting.
The planetar's spellcasting ability is Charisma (spell save DC 20).
The planetar can innately cast the following spells, requiring no
material components ...(MM. p.17)
- In the True Polymorph case, the rules don't expressly forbid spells
in the new form, nor do they state that prepared spells are lost,
nor that they are replaced with the new form's spell casting
feature. Your DM will have to rule on whether or not the Innate
Spell casting feature of the Planetar, or the Arcane prepared spell
casting class feature of the Wizard, will be operative while in the new
form. It would be reasonable to rule that the wizard's prepared spells to remain since the polymorphed creature
retains its personality and alignment. (PHB p. 283)
... and who you are is a wizard. If the ruling was that the new form has innate spell casting of a Planetar, it would make sense that this replaced the prepared/Arcane spell casting of the wizard. (At that level, this could represent a heck of a loss if early in the spell casting day, but a real bonus if most other spells have been expended ...) The replacement of the mental statistics does not, per the spell language, necessarily change "who you are" while transformed under this spell.
... can't speak, cast spells, or take any other action that requires
hands or speech unless its new form is capable of such actions.
The Planetar is capable of such actions.
Who is really in there? The Prince turned into a frog, or is that just a frog?
Note: if the wizard and a Planetar are not the same alignment (LG) this could pose another problem (that probably needs its own question). A simple approach would be that the Planetar's spells are not available -- but that depends on how your DM works with alignment at your table. Arcane spells, and the ability to use them, are not alignment dependent. The Planetar's spells, as they are innate to that creature, appear to be alignment dependent.
As angels are "formed of the divine essence of benevolent gods ...
they are "the embodiment of law and good" (M p. 15)
Best Answer
Okay - Lots of questions means lots of sources. So lets first look at the main spell, simulacrum.
So the key points are that it's a weaker duplicate of the wizard, and cannot learn.
Now lets see what it takes for a wizard, to cast a spell. Under the general rules:
So wizards prepare spells, and we'll look at that in a moment. First, take a look at ritual spells since that will come up shortly.
So wizards can cast ritual spells, but they do it differently than others. So let's see what it says about wizards casting spells, both normal and ritual.
So those are the wrinkles to all of this. Now lets look at the questions again:
1. Prepare new spells after a long rest?
For wizards, to prepare a new spell, requires studying and memorization. This reads to me as "learning" which a simulacrum cannot do. So there is no way for a simulacrum to learn a new spell, only use the spells in its head at time of creation.
2. Cast spells as rituals?
There is nothing stopping it from reading directions from the book--assuming a simulacrum can read your notes. But since you wrote the notes in the first place, and the spell description says that it is the same as the original and has all the same stats, I would say that reading your own handwriting would qualify. And as a bonus, since rituals don't use spell slots, you can cast them repeatedly as a simulacrum cannot regain spell slots.
3 .Copy new spells into the book?
4. Copy their currently prepared spells into a new book that is then usable by the original wizard?
These I am lumping into the same category. The spell states that the simulacrum "cannot become more powerful". Adding new spells to a spellbook, whether into the existing spellbook or in creating a new one, would fall into the realm of "more powerful". However, since the simulacrum cannot actually use the new spell (can't learn it), its up to the DM to decide if that would make the simulacrum more powerful or not.
To the other point, if a simulacrum is tasked with copying a spellbook, they would use the same notation as the original. In fact the PHB states:
The simulacrum would copy the spellbook using the same notation as the original so they would not be able to create a spellbook that the original could not read.
So the short answer is, the simulacrum can cast rituals so long as they have the spellbook handy. They cannot change memorized spells ever. And copying spells is DM interpretation.