Context: In addition to numerous articles on mathematics, Whitehead wrote three major books on the subject: A Treatise on Universal Algebra (1898), Principia Mathematica (co-written with Bertrand Russell and published in three volumes between 1910 and 1913), and An Introduction to Mathematics (1911). The former two books were aimed exclusively at professional mathematicians, while the latter book was intended for a larger audience, covering the history of mathematics and its philosophical foundations. Principia Mathematica in particular is regarded as one of the most important works in mathematical logic of the 20th century.

Question: What did Whitehead publish numerous articles about?
Answer: mathematics

Question: How many books on mathematics did Whitehead write?
Answer: three

Question: Who co-wrote Principia Mathematica with Whitehead?
Answer: Bertrand Russell

Question: Which of Whitehead's books is known as one of the most important works in mathematical logical?
Answer: Principia Mathematica

Question: When did Whitehead write his first book? 
Answer: 1898

Question: What was Whitehead's first published book on mathematics?
Answer: A Treatise on Universal Algebra

Question: With what mathematician and philosopher did Whitehead collaborate to write Principia Mathematica?
Answer: Bertrand Russell

Question: Who was the intended audience of Whitehead's first two mathematics books?
Answer: professional mathematicians

Question: What was Whitehead's final book on mathematics?
Answer: An Introduction to Mathematics

Question: What is the significance of Principia Mathematica currently?
Answer: regarded as one of the most important works in mathematical logic of the 20th century

Question:  What was Whitehead's first unpublished book on mathematics?
Answer: Unanswerable

Question: With what mathematician and philosopher did Whitehead reject to write Principia Mathematica?
Answer: Unanswerable

Question:  What was Whitehead's last unpublished book on mathematics?
Answer: Unanswerable

Question:  What is the insignificance of Principia Mathematica currently?
Answer: Unanswerable

Question: Who was the intended audience of Whitehead's last mathematics books?
Answer: Unanswerable