Digital Macro & Close-up Photography (Revised and Expanded Edition) (7AD). Ammonite Press; Revised edition edition. Available at: https://www.amazon.co.uk/Digital-Close-up-Photography-Revised-Expanded/dp/1907708766/ref=pd_sim_14_4?ie=UTF8&dpID=51oRcOnYvmL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&psc=1&refRID=WJ4YPP5PWB00G70VCVM2.
Ross Hoddinott (no date) The Digital Photographer’s Guide to Filters. David & Charles Publishers.