Hi. I have read The Palace of Illusions. I am yet to read Karna's Wife as I have heard rave reviews of that book.
The Palace of Illusions may be fictional but it helped me understand a minor's point of view in Mahabharata- Karna's. I sought to understand him better and if, like me, you have become a Karna fan, May I suggest you to read Rashmi Rathi (If you understand Hindi, that is.) It is entirely Karna's point of view, in verse form and amazingly well-written. After the palace of Illusions, I have started shipping Karna-Draupadi really hard and I have somewhat become an anti-Pandava, I have humongous amount of grudge against them and against the injustice done to Duryodhana et al. After the Palace of illusions, my firm belief that the Pandavas were no goody-two shoes has strengthened further. It was interesting to know Bishma Pithama's previous life's story too.
Divakruni is a good writer, brilliant narrator and to me, justifies Draupadi's point of view well, though I would have not minded more length, it was a bit incongruous in some parts. My teacher used to condemn the book and asked us never to read it, I went ahead and liked it.
But, the only point not to be forgotten is that it is HIGHLY fictionalized. :-)