Not a fan of "there is a house inside mummy's tummy", myself..
mind you I have yet to find s really good one, and I have been looking since I fell pregnant with baby number 2. We ended up getting 'where did i come from?' and just sort of 'self-edit' when we read it to the kids. I have found most of the books geared towards toddlers and young kids doesn't really explain anything, while books like 'where did I come from?' explain too much.