I've just got a quest where the librarian in the mage's guild wants you to get the book "The Last King of the Ayleids". I already found this book when exploring and dungeoning and have been unable to remove it from my inventory. Now the quest won't finish as the librarian doesn't take the book off me... any ideas... bugged I reckon?

Me too. On top of that I found another book called "Last King of the Ayleids" (same author), that he bought but didn't recognize either.

I'm not positive but I suspect the bug involves the quest NOT being active when you find the objective. Amren's Family Sword (from Whiterun) either wasn't spawned
when I first cleared the bandits out (or I accidentally sold it) - but after several days real time they (and it) had re-spawned when I visited again, this time with the
quest active.

I have had the same problem with several fetch quests. I have the item they want, so I can't "find" it again but they will not take it. Or, they will recognize that I have the item, give me the reward and the quest will end but I still have the item stuck in inventory as a quest item. I know of no workaround.

The only bugs I have had is where I have already taken the item (probably stolen it) and find the quest later, I can complete the quest, the item respawns and it isn't removed from my inventory. So I can get 2 of the item...

Prime example is that staff of arcane *insert text here* in winterhold. Find a 2500septim staff, you bet I am going to steal it as fast as lightning.

A simple fix is using the console commands to give yourself the item that they want. Then after they accept it use the console again to remove the unwanted quest items from your inventory. Another option is to use the console to skip ahead a step in the quest so they react properly. I would definitely try this if I had these issues. Sure, you could cheat and do this all the time, but that's taking away from the experience. This should only be used when the game is acting up. Make sure it is the game and not you.

I have the same problem and that solution is great, except if you are one of the poor guys who are stuck playing RPGs on consoles like the Xbox and PS3:( guess we are screwed waiting for a patch that then Microsoft has to agree to release.