Sol Ut Press launched a new site and is giving away two +300 page books for the month of April, but the PDFs are up already. The site has given these away before. They're good books for the beginning or intermediate player.
Sound the Trumpet: How to Blow Your Own HornSound the Trumpet: Free Stuff
Basic Music Theory: How to Read, Write, and Understand Written MusicSol-Ut Press: Free Stuff
apparently there's also a Podiobook of the trumpet book you can subscribe to on iTunes if you're into that:
Sound the Trumpet: How to Blow Your Own Horn