Land becomes one of the most valuable resources in the game.  Early in the game it is locked by level, later in the game it is very expensive.  It is generally best to upgrade to accommodate growth, rather then add.  Always upgrade houses rather then add another one - ignore the book.  I have consulted on many farms where we could find the equivalent of 2 expansions just by reducing the number of houses.  Buildings tend to be the most efficient (workers/product) at level 1 and 6.