This last subtopic of shapeshifters, I must admit, I have never read in paranormal romance, though I would be eager to get my hands on a book if one does exist. I'm talking about shifters who can change their appearance to that of any human. I'm talking about changelings, doppelgangers, if you will.
I've read about this once in the classic science fiction series Dune by Frank Herbert. In this series, he wrote about Face Dancers which were able to shift their facial features to resemble any person they liked.
Similarly if you've watched the popular TV series Supernatural, you might have seen a shapeshifter change its appearance by morphing the top layer of its skin to resemble the person it wishes, and later shedding its skin.
A Doppelganger is considered in mythology to be an apparition or double of a person. It has its own mythology surrounding it, claiming that every person has a ghost-like counterpart, and this counterpart is considered to be evil. Here, I used the term in the common urban disambiguation, meaning that it is literally a person's look-alike. If you want to learn more about the original myth surrounding the doppelganger, take a look at the Wikipedia page, this website, or this Fact or Fiction? website.
A Changeling in mythology refers to a fairy spirit who, at birth, was switched with a living child. This spirit can take on the physical appearance of the child and is considered to be evil. More information about changelings can be found at the Wikipedia page, this website, or this website about German Changeling legends.
Visit the Wikipedia page about shapeshifting to learn more about it in general.
Don't forget to stop by this Wednesday to read a review of Mary Winter's Twin Spirits!