There are lots of books about humanism, but here are the introductory texts recommended by Humanists UK:

• What is Humanism? by the Humanist Philosophers Group
• On Humanism (Routledge) by Richard Norman
• Humanism: A Very Short Introduction (Oxford University Press) by Stephen Law
• Humanism: A Beginner’s Guide (Oneworld) by Peter Cave
• Humanism: Beliefs and Practices (Sussex) by Jeaneane Fowler
• The Good Book: A Secular Bible (Bloomsbury) by AC Grayling
• The Wiley Blackwell Handbook of Humanism (Wiley Blackwell) edited by Andrew Copson and AC Grayling (two chapters can be downloaded here: What is Humanism? and Life without meaning?)
• Humanist Anthology (Rationalist Press Association) edited by Margaret Knight (an excellent source of quotations and humanist thought from Confucius and the Ancient Greeks to today)
• Humanists UK’s free downloadable ebooks offer a humanist approach to life’s big questions: Living Well by Richard Docwra and Being Good by Richard Norman

You might also be interested in an analysis of the positives of atheism: Atheism: A Short Introduction (OUP) by Julian Baggini

For more on secularism see Secularism: Politics, Religion, and Freedom (OUP) by Humanists UK CEO Andrew Copson

And for young adults we recommend The Young Atheist’s Handbook by Alom Shaha.