There are some books on sailing Mirror dinghies that have been published in the UK over the years. In order of age, they are:

• "Sailing The Mirror" by Roy Partridge, published by Fernhurst Books ISBN 0 906754 01 1 published in 1980 and no longer in print. This is a "How to Sail" book specifically for Mirror Sailors. Over 120  black and white photographs and drawings show you in simple logical steps how to rig, sail, tune and race your own boat. No prior knowledge of sailing is required. Roy Partridge started sailing Mirrors in 1964 and has won UK National, European & World Champions.
• "Mirror Racing" by Guy Wilkins, published by Fernhurst Books ISBN 0 906754 45 3 published in 1989 and no longer in print. This book is a guide to making the most of this spectacularly popular boat. Using photo-sequences and diagrams, Guy Wilkins describes how to make your Mirror the quickest on the water - if necessary by building a new one - and explains how to fit it out and sail it fast on all legs of the course and in all conditions.
• "The Mirror Book" by Peter Aitken, published by Wiley Nautical ISBN 978-0-470-51938-7 published in 2008 and still in print, cost £14.99 . Packed with practical advice and illustrated with step-by-step photographs, this book teaches you how to sail the UK's most iconic and best loved dinghy. Learn all the essentials, form sailing theory and rigging to faster upwind and downwind sailing. Peter Aitken, the UK National Mirror Coach in 2008 and Tim Davidson, a successful racer, reveal the basics and show how to outsmart the opposition. This book includes Bermuda and Gunter rigs, centre mainsheets & flyaway pole systems.

The books which are no longer in print do turn up in used condition from time to time on sites like Amazon, or in local second hand bookshops.
Needless to say, a book written in, say 1980, will reflect the rules and state of Mirror sailing in 1980. Things move on, ideas change, rules change, so the older books will be of little use if you want advise on rigging a bermuda mast on a boat with a centre mainsheet or a flyaway spinnaker pole system. However, they do still contain some useful advise on basic fittings (either rig), setting up a gunter rigged boat & racing basics.