Unsolved Problems in Number Theory (Problem Books in Mathematics / Unsolved Problems in Intuitive Mathematics)

From the reviews of the third edition:

This is the third edition of Richard Guy's well-known problem book on number theory ... . The earlier editions have served well in providing beginners as well as seasoned researchers in number theory with a good supply of problems. ... many of the problems from earlier editions have been expanded with more up-to-date comments and remarks. ... There is little doubt that a new generation of talented young mathematicians will make very good use of this book ... . (P. Shiu, The Mathematical Gazette, Vol. 89 (516), 2005)

The earlier editions of this book are among the most-opened books on the shelves of many practicing number theorists. The descriptions of state-of-the-art results on every topic and the extensive bibliographies in each section provide valuable ports of entry to the vast literature. A new and promising addition to this third edition is the inclusion of frequent references to entries in the Online encyclopedia of integer sequences at the end of each topic. (Greg Martin, Mathematical Reviews, Issue 2005 h)