If you choose e-books, you can consider two very cost-effective K3 and K6 from Taipower, and the price of K3 is around 1450.
As for MP4, there are many models that support PDF, such as Meg HD8800. The latest firmware can support both PDF and CHM, and some Onda models such as VX575 Full HD also support it. However, it's best for MP4 to read e-books with shuttle operation or touch screen, and the buttons are easy to break.
As for the mobile phone, it is the least recommended, because the screen is small and the supporting format is not as cost-effective as MP4.