I have just finished reading 'MAKING SIMPLE MODEL STEAM ENGINES' by Stan Bray (ISBN 1 86126 773 and it contains several boiler designs with drawings and plenty of explanations. To a newbie like me, it seems great.
Hope this helps,
See this post (today): "Model Stationary and Marine Steam Engines" by K N Harris. That book is a free d/l, link in the post, AND - there´s also a free d/l for "Model Boilers & Boilermaking", by the same author. Both are excellent books.