Preview the handbook in Google books where you can also search the whole contents.
The printed version of the handbook is available for purchase at https://www.createspace.com/4039211
http://www.amazon.co.uk/ and other Amazon sites.
If you belong to the COST action, use the discount code 4V6CXNY4, and you will get the handbook for less than 20 USD plus postage. Normal price is 26 USD, 17 GBP or 23 EUR.
The handbook is also available through Amazon (USA, UK, DE, ES, FR, IT). However, the discount code is valid only for direct purchases through CreateSpace at the link above.
The open-access PDF file of the handbook is available from the repository of the University of Helsinki called Helda. The top link below is to a temporary location at DropBox (bookmark this page, not the link to a high resolution file). It is a large (55 MB) and slow download, but figures are at the same resolution as in the printed copy. The second link is to a version of the file optimized for on-line use with figures at low resolution. This is a smaller file (8.5 MB).
These links point to the 2nd corrected printing, released on 1 February 2013.
Handbook as high resolution pdf file (ISBN 978-952-10-8363-1) size 36 MB (no cover).
Handbook as reduced resolution pdf file (ISBN 978-952-10-8363-1) size 8.5MB
This link still points to the 1st printing, released in November 2012.
Permanent URI to the open-access repository of the University of Helsinki: http://hdl.handle.net/10138/37558
Aphalo, P. J.; Albert, A.; Björn, L. O.; McLeod, A.; Robson, T. M.; Rosenqvist, E. (eds.) 2012. Beyond the visible: A handbook of best practice in plant UV photobiology. COST Action FA0906 UV4growth. Helsinki: University of Helsinki, Department of Biosciences, Division of Plant Biology. ISBN 978-952-10-8362-4 (Paperback), 978-952-10-8363-1 (PDF). xxx + 176 pp.