I remember reading this book in the around 2005 perhaps? Although I think it was published earlier of course. Anyway. It was about this alien made of pure energy who absorbed living matter in order to shapeshift. She was good - and traveled to planets absorbing cultural information and learning about different species on colonized planets. She was already hundreds of years old, and the rest of her "family" of such beings had been destroyed by something. She became friends with a human male, who I could SWEAR was named Paul, and he helped her to hide from people and such. There are multiple books - like it was a series. And if I had to hazard a guess I would say it was from Tor Books. But don't quote me on this. It's been driving me crazy for WEEKS. I just can't figure out what it was. Plz help, O Mighty Internet!
The book you are looking for is Beholder's Eye by Julie E. Czerneda. It is the first book of the Web Shifters trilogy.
The alien (Esen-alit-Quar, Esen for short) is the youngest (and last surviving) member of an alien race that is energy based and can take on any shape they have memorized/studied.
At the start of the book she has been living on a planet (as an animal) observing the (intelligent) natives for a number of years. She is captured and imprisoned with a human (Paul) and in order to escape she has to reveal her special nature to Paul.
As the series continues Es struggles with keeping her alien nature hidden while trying to find out what happened to the others of her kind.