"Combinatorics: A Problem-Oriented Approach" by Daniel A. Marcus
"Graph Theory: A Problem-Oriented Approach" by Daniel A. Marcus
"Lie-Groups: A Problem-Oriented Introduction Via Matrix Groups", by Harriet Pollatsek
“Number Theory Through Inquiry”, by David Marshall, Edward Odell, and Michael Starbird
Distilling Ideas: An Introduction to Mathematical Thinking by Michael Starbird and Brian Katz
“Exploratory Examples for Real Analysis”, Joanne E. Snow & Kirk E. Weller
“Explorations in Complex Analysis”, Michael Brillslyper, Michael J. Dorff, Jane M. McDougal, James S. Rolf, Lisbeth E. Schaubroeck, Richard L. Stankewitz, & Kenneth Stephenson
“Exploring Advanced Euclidean Geometry with Geogebra”, by Gerard Venema
“A TeXas-style introduction to proof”, by Patrick Rault and Ron Taylor.
Coming soon. Preliminary copies available through the authors.