I haven't read Ender's Shadow, but I've heard it was quite good from a few people. On the other hand, I have read Speaker for the Dead and Xenocide. I may catch some flack for this, but I'd suggest leaving those alone. For all that they're supposed to be direct sequels, they really didn't feel like the same series at all. I haven't read Children of the Mind, so I couldn't say one way or the other on that front.