About the Istari, the chief source, outside LotR and
Of the Rings of Power in the 1977
Silmarillion, is
The Istari in Unfinished Tales (which also deals with the remaining two Wizards beside Gandalf, Saruman and Radagast), plus some references, mostly to Gandalf, in the Prof's Letters.
About Melian, I think
Gorthaur refers to the passage in the
Silmarillion (
Of the Ruin of Doriath) which says (more or less - I'm translating back from my German edition, so this is not a precise quote):
'For Melian was of the divine kin of the Valar, and she was a Maia of high power and wisdom; but out of love for Elwe Singollo she had taken the form of the Elder Children of Ilúvatar, and in this marriage she was bound to the flesh of Arda. In this form she bore Lúthien Tinúviel to him and in this form she gained power over the substance of Arda.'
I have a notion Tolkien himself may have compared Melian's incarnation to Morgoth's in
Morgoth's Ring (History of Middle-earth X), but I don't have the book with me to check it. Maybe someone else could look it up for us?
Form, thanks for improving on my post (and do I sense a mild correction of my flippancy there?
)! Of course this thread has potential for interesting speculation, I didn't mean to deny that.
In the case of the Istari, it's difficult to tell where 'forbidden' ends and 'incapable' begins, or whether both were actually two sides of the same coin. We never
see Saruman neglecting the conditions of his contract so far as to display his full Maiarin power - does that mean he still felt bound to the letter of it, if not the spirit, or does it mean he couldn't if he'd tried? On the other hand, did Gandalf use his full power in his unwitnessed battle with the Balrog? I'm afraid we'll never know.