In addition to Bhante Henepola Gunaratana‘s excellent meditation manual Mindfulness in Plain English, which is available in print from Wisdom Publications or as a PDF file online (http://www.urbandharma.org/udharma4/mpe.html), Thanissaro Bhikkhu has a very thorough book available free of charge on the Metta Forest Monastery website in either PDF, epub, or mobi file.
With Each & Every Breath: A Guide to Meditation. (e-books revised Oct. 19, 2013) A new breath meditation manual by Thanissaro Bhikkhu drawing on two sources: the Buddha’s own set of instructions on how to use the breath in training the mind, and Ajaan Lee Dhammadharo’s method of breath meditation — which builds on the Buddha’s instructions, explaining in detail many of the points that the Buddha left in condensed form. A page has been added to this site with download links to all of the .mp3 dhammatalks referenced in this manual: With Each & Every Breath audio files.
The link to With Each and Every Breath is http://www.dhammatalks.org/ebook_index.html#eachandeverybreath