I gave the runed lexicon to Septimus Signus, after Hermaeus Mora killed him I took back the runed lexicon. Is there any point or use for keeping this item, other than some slight sentimental value?
There is no use for the Runed Lexicon after the quest is complete. It is supposed to disappear after the quest is complete but there is a bug which may mean it does now, If you loot it from Septus' corpse, you may not be able to drop it. If you are playing on the PC, you can get rid of it with the following console command:
If you're on a console, there's no known fix for the bug.