Ender's game is a mixed bag… He wrote the books in that series over a very long period and they change in style quite a lot, they also change in meaning and intent. By “Ender's Shadow” he's basically re-written the whole meaning of the first one. It's a bit lame.
Not like the Dragon and the George series by Dickson where he wrote those novels over an even longer period, but even with something like 20 years between the first and the second, it's amazingly consistent.
Don't get me wrong, Orson's Ender's books aren't bad, and growing, changing, and evolving is great in an author, but it's very transparent here:
-Ender's game is very much directed at teenagers. It's a great story, but you can see it has that level of understanding in mind. I first read it in highschool and loved it, then read it a few years ago and still liked it but the limitations were more apparent. Better as a stand alone book)
-Speaker for the dead is VERY different in style. This isn't a compelling action story like Ender's game, it's really a whole different story, just coincidently with 2 of the same characters and sort of a similar universe. It's a far more adult, introspective, thoughtful story.- My fave of the bunch BTW and better as a stand alone book.
-Xenocide, ostensibly a direct sequel to Speaker for the Dead. It's AMAZINGLY weak. And suddenly you have all these silly metaphysical aspects added into the story which weren't there in the earlier books. By this time Card has tried to take the idea in yet ANOTHER new direction, but in such a ham-fisted way. It wouldn't have been so bad if it wasn't supposed to be connected to the other two stories (for the concepts introduced), but this one could not stand alone, and even with the others for support it fails.
-Ender's shadow… (I think it's called that). This is the last one in the series that I read or will ever read. In this story Card goes BACK again to the original ideas in Ender's game, with the younger focus and all, trying to recapture the magic… But being as this is Orson Scott Card here, he has to change the theme and the story yet again. Ender's shadow attempts to change the tone and re-write the story of the original novel with a darker, grittier feel. Unfortunately it's not only not as good a story as the original, the changed focus and different perspective serves to cheapen Ender's Game.
So my recommendation would be to read Ender's Game, then give it a few months and a few novels in between before reading Speaker for the Dead. -That way your mental palette is cleansed and ready for the new and quite different treat. :)
Ignore the other books.
I disagree with you about xenocide…if you read speaker for the dead and then xenocide, theres alot of continuity between those…I mean, he was on a completely new planet, so of course thered be a new direction. I would wait between enders game and speaker for the dead, and then read speaker and xenocide back to back.