I think it could have been written better. I think it needed more details. Like I couldn't understand how Elena died as a human changed into a vampire came back as a ghost then came back to life in vampire form. I was so confused. But it had a good story line. I just needed more detail. i love Damon! i always fall for he bad guy!
LJ Smith has started writing a new trilogy for the series called The Return. The first book was just published in February and is more so to be from Damon's point of view. I just picked it up but I plan on re-reading the original four books before I start on it.
She became a vampire because she had swapped enough blood between Stefan and Damon to have the vampire "venom" in her to change her when her heart stopped beating.
She became a ghost because she had a strong will to watch over her friends and loved ones after her death.
As far as coming back in vampire back when resurrected.... I thought she came back in human form?? If she did come back in vampire form it was probably because it was her last living form.