iPad's iBook app doesn't offer a simulated page surface as some dedicated eReaders such as Kindle do, so it's important that you make the reading experience as visually comfortable as possible by adjusting the brightness and font settings.
Adjusting brightness
iBooks offers an adjustable brightness setting that you can use to make your book pages comfortable to read. Follow these steps to make an adjustment:
With a book open, tap the Brightness button.
Tap and drag the slider to the right to make the screen brighter, or to the left to dim the screen.
Tap anywhere in the book to close the Brightness dialog.
You have to experiment with the brightness that works for you. It's commonly thought that bright white computer screens are hard on the eyes, so going with the halfway default setting or below is probably a good idea.
Changing the font size and type
If the type on your screen is a bit small for your taste, you can change to a larger font size, or choose a different font for readability.
With a book open, tap the Font button (it sports a small and a large capital A).
In the Font dialog that appears, tap the A button on the left to use smaller text, or the A button on the right to use larger text.
Tap the Fonts button.
A list of fonts appears.
Tap a font name to select it.
The font changes on the book page.
Tap outside the Fonts dialog to return to your book.
Some fonts appear a bit larger on your screen than others because of their design. If you want the largest fonts, use Cochin or Verdana.