The guide book
‘Bhutan: a trekker’s guide’
(ISBN 978-1-85284-553-7)
is written by Bart Jordans.
The second edition is published in January 2008 by Cicerone Press, an outdoor specialised publisher based in the UK with nearly 300 different guide books.
It is available directly through them, through several web book selling services or at any outdoor bookshop.