The answer to that question depends a bit on what type of reading pen system you are using, as different companies use different technology. Products such as LeapFrog Tag and VTech's Bugsby Reading System use specially-designed books with their reading pens. In order to access more titles, you will need to purchase their books, most of which are targeted towards younger or early readers.
The benefit of books like these is that the text is typically read by a human voice, rather than a computerized voice. Children can also click on different icons within the text to get more information or sound effects while reading. The disadvantage to such products is that they can only be used with a limited number of titles and so may not be a good solution for a student with disabilities who needs to access a wider array of books.
You may also look at the different types of reading pen/scanners available — many of these tools you use like a highlighter, running over printed text and getting instant speech feedback. While most of these pens are not appropriate for reading an entire book, they can serve as a valuable support for difficult words, definitions, and pronunciation.