As part of a new training programme in my company I tried several references for preparing for my APMP qualification. This was by far the best.
APM BoK is a good starting point and reference.
APM Body of KnowledgeAPMP Primer provides good additional material and some exam guidance.
An APMP PrimerThis book though is a one stop shop that is a good reference, provides additional material, and also provides a logical training approach with exam guidance.
My only comment is that the book is a bit pricey, however the return on investment for me was worth it, especially as it links to some free web content and support.
I suggest ignore the other books and buy this one, or if you are feeling rich buy them all as having the cross reference has allowed me to understand the subjects more deeply.
Happy studying.
Rich