Sauron is able to retake form after the RIng is removed from him because even when he doesn't have the RIng, the power of the ring "existed and was in 'rapport' with himself: he was not 'diminished'."
It seems that when a maia dies, his/her spirit floats around harmlessly throughout Arda. Sauron's spirit is said in the Valaquenta to have followed Melkor's into exile in the Void.
I don't recall any specific good maiar who have died.
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"He was sent by a mere prudent plan of the angelic Valar or governors; but Authority had taken up this plan and enlarged it, at the moment of its failure."
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