I teach piano and am using this book for my students who approximately grade 2 standard. It will take them a few weeks to learn one piece. Each piece is 2-3 pages long. Some of the adaptations are better than the others, but at least this book gives a piano student an opportunity to learn some fun music at a relatively early grade. The pieces are all in simple key signatures (maximum 2 sharps/flats).
I have purchased many of the books in the Really Easy Piano series and find them to be generally good value for money and suitable for students of all ages at the early grades. If you are deliberating which one to buy and you don't have a teacher like me who has a lot of them that you can view, then see if you can find out what songs are in the books and pick one with songs that you are interested in learning. That is the one disappointing thing with buying manuscript from Amazon - you can't see the list of pieces before you buy the book! 21st Century Hits seems to generally have pieces that were in the charts a few years ago so will not be suitable for the youngest students, since they may not recognise the songs.
The pieces in 21st Century Hits are:
Beautiful - Christina Aguilera
Born to Try - Delta Goodrem
California - Phantom Planet
Can't Get You Out of My Head - Kylie Minogue
The Closest Thing to Crazy - Katie Melua
Crazy - Gnarls Barkley
Don't Know Why - Norah Jones
Don't Stop Moving - S Club 7
Eternity - Robbie Williams
Evergreen - Will Young
If You're Not the One - Daniel Beddingfield
Mad World - Michael Andrews feat. Gary Jules
Oops!...I Did It Again - Britney Spears
Put Your Records On - Corinne Bailey Rae
Round Round - Sugarbabes
Sing - Travis
Somebody Told Me - The Killers
Somewhere Only We Know - Keane
Star Girl - McFly
This Year's Love - David Gray
Vertigo - U2
Wires - Athlete
Yellow - Coldplay
You're Beautiful - James Blunt