There have been a few posts recently from people asking for help on building shelters, high seats etc.

I'd recommend this - it covers building a range of structures, from tree houses, towers, sheds, huts, to log cabins. Mostly from from timber you would find in the woods. It also shows you how to work and join logs etc

Lot of good ideas.

Rustic Retreats - A Build It Yourself Guide. David & Janie Styles (Storey, 1998). ISBN-10 : 1-58017-035-8.

About a tenner.