Here are additional mathematicians thoughts.
S. Ulam, Adventures of a mathematician.A recollection of his life, from Lwow to Los Alamos. I am linking to excerpts. The book is still available for purchase.
Advices to a Young mathematician, a collection of advices and anecdots by M. Atiyah, B. Bollobas, A. Connes, D. McDuff and P. Sarnak.
A. Borel, Art and science (Math. Intelligencer vol.5 1983, translation from german). A text for a general audience about the relationship between art and mathematics.
R. P. Langlands Is there beauty in mathematical theories?, this text is actually about number theory, old and new.
T. Gowers The two cultures of mathematics, another take on the dichotomy between problem solving and theory building.
A. Connes A view of mathematics, a torough exposition of A. Connes'philosophical stance about space and physics. Targeted at a scientific audience.
D. Mumford, the dawning of the age of stochasticity, from algebraic geometry to statistics.
Y. Manin, Interrelations between Mathematics and Physics, on the divergence between mathematics and physics in the XXe century.
M. Gromov, ergobrain, one of the most surprising inquiry about life and mathematics.
I end that list with a text from a french mathematician about the future of mathematics: Poincare, l'avenir des mathematiques.