The Ugley Vicar has links to the free online text of 2 classic books on mission, by Roland Allan:
Missionary Methods, St. Pauls or Ours?
and the wonderfully titled
'The Spontaneous Expansion of the Church and the causes which hinder it'
With the crumbling of the SPCK chain in the UK, second hand section and all, it's good to find stuff like this on the web instead.