Beware if you were like me and bought levels as they were introduced (levels 1-3). The least expensive way to add on is you have to rebuy levels 1-3 and buy all the levels. Otherwise price-wise you are hosed.
They do not give you any credit for being a loyal customer customer (In addition to levels 1-3 of endless reader, I had previously purchased endless alphabet and endless numbers).
Many other apps give you some credit for the apps you bought before.
The app is great, but now the way they do business.
Dissapponted Grandmother about Endless Reader