Most readers have similar functions-change text size/color, bookmarks, annotations, can switch to white text/black background

• iBooks (free) Be sure you have the latest version (iBooks 2) in order to be able to access newer etextbooks. Apple's answer to Amazon and Barnes and Noble's digital stores- w/built in dictionary and access to Project Gutenberg (books in the public domain) iPods/iPads also include voice-over option (found in settings)
• Kindle (free) can only download books from Amazon
• Pdf-Notes Free for iPad (free) Pdf reader/viewer-allows you to annotate. For a very good paid pdf reader, check out GoodReader ($4.99) 
• Many newspaper readers (free-requires wifi) USA Today, NY Times,  Chicago Tribune and MANY more international papers 
• Dr. Suess -Various Stories ($3.99 ea.) 
• Bartleby's Book of Buttons (Vol. 1 and 2-each .99)  Lots of buttons to push while reading.
• Grimm's Rapunzel-3D Interactive Pop Up Book ($3.99) REALLY a beautiful, fun book!
• Britannica Kids: Volcanoes ($4.99) Beautiful, interactive textbook--very well done. 
• CK12 epub textbooks-California initiative. Completely customizable. Find epub versions through iBooks. Text only--not interactive
• Nook (Barnes & Noble Books only) Free