Have you ever had a package of books say it was delivered at your house, but when you got home nothing was there? I was supposed to have had two packages of books yesterday that said delivered, but when I got home from work at six there was nothing on my front door step.  Has this ever happened to anyone?

Yes, and I contacted the USPS. I also asked my neighbors.  The actual item was delievered a few days later. No reason was given, they did say they'd look for it.

Several times through the years. Some P.O. will scan as 'delivered' when it's reached the last P.O. station on the route to final delivery. 

Sometimes I've noticed it early in the morning and it was delivered the same day. But a few times it took a couple of days.

Once, yes.  Turns out the box was delivered to my neighbor who brought the box to my house later that evening.  It ended up on my porch & had obviously been opened already, but all the books were there so I was glad for that, but it concerned me that other mail of mine was going to my neighbor's homes too.

Oh yeah. Once I waited for 16 days for the scanned "delivered" package to turn up at my house. No clue where it was all that time. Probably lost in the mail truck.

Also be aware that some folks do go up onto your porch and they steal packages.  You may want to call the Post Office to see if they did in fact deliver it and if they did, you may want to provide a safe secure place for parcels to be left.  Be sure to file a mail theft form letting them know that it is occurring.  I have a tall wooden box which does open at the back, facing my house.  It is right next to my front door and my carriers drops parcels in there.  You cannot see them even if you come to my door.  Please let your carrier know if you are having parcels disappear.  Try to work something out between the two of you.  Postal Inspectors need to know when mail is being stolen. They can then set up stakeouts and traps to try and capture these criminals so if no one reports anything, the theft continues as usual and the crooks are happy.

Can you ask a good neighbor to accept your parcels?  Someone you can trust.  

Hope your parcels are found and soon.  Good luck.