C. K. Ogden 
Nature Of Mathematics Ilphil28 [PDF ebook] 

First published in 2000. This is Volume I of eight in the Philosophy of Logic and Mathematics series. Written in 1933, in The Nature of Mathematics offers a critical survey the author seeks to present a considered critical exposition of Principia Mathematica and to give supplementary accounts of the formalist and intuitionist doctrines in sufficient detail to lighten the paths of all who may be provoked to read the original papers.