The URL is http://www.mobileread.com/forums/sho...1&postcount=34
Just click the little post count number (#34) in the upper right corner
In the URL you can find the post number, but this linking does not use the thread number.
Edit:Eventually, if you want to use the thread number and know the page of the thread the post is on, you could use http://www.mobileread.com/forums/sho...essage_1697211
Note that the message count is the post number from the first URL, but that the linking would break if someone changes the behavior of mobileread to show more/less posts per page. The only reliable way is to use the first method