I wish to buy an iPad to have some digital interactive books. The requirements for the books are: "To view this book, you must have an iPad with iBooks 3 or later and iOS 5.1 or later".
I don't wish to buy a new iPad (low on cash), and wish to buy a second hand one, as I'll be using the iPad only for using those digital books.
I know nothing about iPads and their different versions. What would be the minimal iPad generation (normal size or mini) I should buy?