I have heard of some other people having that problem, but it's quite rare (maybe because most people aren't under 15 when the attempt it), and may have been fixed by one of the patches. Before trying much else I'd make sure you have your game updated to the most recent version.
People have seemed to be able to solve the bug by reloading the game and waiting until level 15 to hand over the Lexicon, but that's obviously a pretty horrible option since you're level 42+. I'm only mentioning it in case if someone else finds this thread that's still under 15.
Your options can depend on whether you have the game on the 360, PS3, or PC. PC players can use console commands to advance through the quest. 360 and PC users have said that they've received the courier and letter by waiting in Winterhold/other major cities). The PS3 seems to be the odd one out, but I'd still try waiting just to see if it works.
I'm sorry if the above don't work, this is a bug that doesn't really have a lot of sure-fire fixes. Let us know what happens, if it doesn't work maybe we can try something else.