NASA has announced the availability of e-books about two of its space telescopes, the Hubble and the James Webb Space Telescope, free of charge.

The e-books -- "Hubble Space Telescope: Discoveries" and "Webb Space Telescope: Science Guide" -- are available at the Apple iBookstore, the space agency said Friday.

The books, written on a high-school reading level, can be viewed using iPads with the free iBooks app, the announcement said.

"These new e-books from NASA will allow people to discover Hubble and Webb in a whole new way -- both the science and the technology behind building them," said Amber Straughn, astrophysicist on the Webb telescope project at NASA's Goddard Space Flight Center in Greenbelt, Md. "They collect all of the amazing resources about these two observatories in an excellent product that I think people will really enjoy."

The e-books are interactive and include image galleries and video, the books' design team said

"For instance, the readers can watch a galaxy collision simulation video, manipulate a telescope model to see it from all angles, or flip through a gallery of planetary nebulae --all right there on the page," Tracy Vogel of the Space Telescope Science Institute in Baltimore said.

Information about how to download the books, as well as non-interactive PDF versions, is available at http://hubblesite.org/ibooks/.