What will be the theme and who will be the guest star on next week's
show. (25th) Is there a reliable source I can check each week?

Dear Nkamon, 

TV.com is a very good source on everything on television. 

They have an episode guide, where they write the expected theme: 
Top 6 Perform
<http://www.tv.com/american-idol/top-6-perform/episode/715845/summary.html>
- episode summary (Classic Love Songs)

With guest stars, it is trickier, but the TV.com forum participants
might be able to help (if not they, probably no-one)

<http://www.tv.com/american-idol/show/11307/forums.html> 

I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it.

As you may know by now, it's Greatest Love Songs and Andrea Bocelli next week.

I don't know of any advance source except for the show itself though.

I hope this helps.

Have a good day.

Yes! And I'm practically giddy about it! I (yes, I'm openly admitting
it) am a HUGE American Idol fan and I'm thrilled with next week's
theme and guest! Woohoo! :)

Sincerely,
Boquinha-ga

Thank you for the rating!