The $25 reward is limited, because after paying for the book, tax and shipping there's not much left for buying tools; if everyone chose that reward I would reach my stated goal but I wouldn't have enough left over to buy the tools I need.
The ready-made houses are limited because of the March delivery deadline. I want to be sure I can make and deliver all the houses I promise on time. If you want a completed house but those slots have sold out, become a $12 backer. I'll put you on my mailing list and when I'm ready to make more houses I'll let you know -- or pledge $100 and up (without being officially in that category), and I hereby promise to make the house for you as soon as I possibly can.