Books written as philosophical argument -- not commentary, not summary, not opinion. Each one takes a question that resists easy resolution and works it through to a position, using reason, evidence, and structured analysis as the primary instruments.
Five books. One argument: that every civilizational failure is ultimately a philosophical failure -- a failure to correctly answer the questions of truth, power, morality, design, and human identity.
Four of the fourteen stages in the Human Transformation System engage directly with questions that are classical philosophy -- existence, free will, personal identity, and the ethics of obligation. These four work as philosophical arguments in their own right.
A single book that does not belong to any series -- and does not need to.