Free e-textbooks

Sociology of the Family (2010) by Ron J. Hammond & Paul Cheney of Utah Valley University, USA.
This book comprises 18 chapters covering aspects such as Communication, Parenting, Divorce and Remarriage. The chapters are very accessible, being quite short and including attractive diagrams and pictures. It lends itself to being dipped-into when necessary, rather than being read cover-to-cover, and if you’re the type who fills in the self-analysis quizzes in glossy magazines, you might want to head straight for the worksheet-assessments at the back!
The book is offered free in a range of formats including .mobi for Kindle and .epub for smartphones, iPad and other e-readers.  

IACAPAP Textbook of Child and Adolescent Mental Health (2012) Edited by Joseph M Rey.
This global publication by the International Association for Child and Adolescent Psychiatry and Allied Professions in Geneva, is intended for professionals training or practising in mental health, with a particular eye to the needs of those in developing countries. It is based on best clinical practice, reflects the latest research, and will be updated regularly. It is a comprehensive and authoritative book divided into ten sections, each including full-colour pictures, references and links to multimedia content. Each section can be downloaded as a pdf and kept.  

Early Childhood in Focus
This series edited by the Child and Youth Studies Group at The Open University reviews the best and most recent research, information and analysis on key policy issues, offering clear messages for for policy-makers, advocates and practitioners. The series, sponsored by the Netherlands-based Bernard van Leer Foundation, features contributions drawn internationally from renowned experts in the field. Currently there are eight volumes, each between 40-60 pages long. The two most recently published volumes cover developmental neuroscience, and health and children’s environments. Each includes full-colour pictures and substantial references. The series is also published in Spanish as La Primera Infancia en Perspectiva. Each volume can be downloaded as a pdf and kept. Copyright © The Open University.

Educational Psychology (2011) by Kelvin Seifert & Rosemary Sutton.
A comprehensive textbook covering topics including student development and diversity, classroom communication and assessment, and much more. 13 chapters, 335 pages and offered in both standard .pdf format and .epub for mobiles or tablets.