good site and getting good hits... but need more improvement.

Dear SEO Fellows...

We are working on www.medicalbooks.com SEO it's a
good site and getting good hits... but need more improvement. we are very
much worry about its Google PR... its Google PR is 5 from last
one and half year... Why is that? And we have added one link
exchange page as well and we also keep looking the ways to
increase link popularity.
Please kindly Reply how to improve its Google PR?
And why its not increasing from 5.

Sorry but this forum is only for questions related to DMOZ.
For questions about Goolge and other SEO related stuff you will have to go to another forum.