I have a couple of questions about these. I've been trying to find answers, but the more I read, the more questions I have.
If I'm flying an aircraft that has only one book in it, and that book is labeled POH, I can assume that the AFM (the only part that's technically required) is included in it, correct?
More or less correct. For context, it's important to understand what a POH is.
A POH, "Pilots Operating Handbook" is a combination of material required by the certification regulations (AFM) and other material that the manufacturer wants you to have. "POH" actually refers to the
format of the book. In 1977, GAMA (General Aviation Manufacturers' Association) published Specification No. 1. It is essentially a consensus among manufacturers regarding the order and naming of the sections of the flight manual and the material that goes into each so that transitions between makes and models would be easier.
Because the AFM is specific to that partical serial number, that "POH" is now anointed and is the only book that can be used in that aircraft, correct?
I'm not sure what you mean by "only book that can be used in that aircraft." Being specific to the aircraft, the POH will include supplements for the operation of N-number-specific equipment. The POH from another aircraft, even the same make, model and year, might not have the information for yours.
If the manufacturer then makes changes to the POH they are putting in brand new aircraft of the same model, are you then allowed to modify the POH in your aircraft to match it, by replace/adding/remove pages?
Not really. There is going to be information in a 2012 C172 with a G1000 panel POH that will be completely inapplicable to your 2002 round-dial C172. That's an extreme example but you will often find small changes between series of aircraft within a model. An more subtle example I can think of offhand is that the pre-1991 Mooney M20J has different performance numbers (and I think some limitations also) than the post-1991 aircraft of the same model.
What does happen is that there may be applicable regulatory or other changes that lead to updates in AFM information. Revisions will be sent out to the owners of affected aircraft. 23.1581(f) requires the AFM to be in a form that allow for the incorporation of revisions and amendments, likely why a POH tends to be in a looseleaf format.
Let's say you can do this, are you required to? Can you legally operate perpetually with the POH/AFM that shipped with your aircraft, regardless of changes the manufacturer makes to the POH they are shipping with new aircraft?
Since you can't just willy-nilly decide to change your AFM, let's change the question to whether you are required to incorporate the revisions and amendments that are made for
your N-Number. You are. Keep in mind that for aircraft subject to the 91.3 requirement for an AFM, what is required is a "
current, approved Airplane or Rotorcraft Flight Manual" (my emphasis).
Just for jollies, let's toss the PIM (Pilot Information Manual) into the mix since some many of us buy one when training or when renting. A PIM is just a term we use for an off-the-shelf manual in the POH format. It looks just like the stock POH for the same aircraft but it is not aircraft-specific (that page with the serial number will be there but the serial number will be blank), does not reflect any later changes or added equipment, and doesn't get updated.