I regularly run across free access to great books online.
The following are a few that have caught my attention recently. I personally love books so I would recommend purchasing each of these for your own library.
Handbook of Distance Education
View PDF here
Bransford et al.
Download PDF of the book or specific chapters
Teaching in a Digital Age: Guidelines for designing teaching and learning
A.W. (Tony) Bates
Download book in various formats
Online Distance Education: Towards a Research Agenda
Teaching and Learning at a Distance
Using Technology with Classroom Instruction that Works
Visual Literacy: A way to teach and learn
Building Online Learning Communities
Know thy impact: Visible learning in theory and practice