This is the link to a pdf about port forwarding on your particular router.
http://files.dlink.com.au/Products/D...Forwarding.pdf
I am not currently aware of any vulnerabilities in the Calibre Content Server. However, your reluctance to open ports is both understandable and prudent. Whenever we allow access from the internet we have to accept that there is some risk involved. I don't believe the risk is a particularly high one in this case, and many people use the content server outside of their home networks. You must of course use your own judgement and of course take responsibility for what you decide.