It's important to teach your child to comprehend complex stories while you're working on building pre-reading and early reading skills. The Biscuit books, while wonderful, aren't exactly rich literature. The vocabulary is kept intentionally simple for early readers. Therefore, it is important that you read books to your child that s/he is clearly not ready to read alone yet.
Why is this type of reading important? For several reasons. One of the most important aspects of preschool development is building a large database of words. Vocabulary development depends on exposure to a rich diversity of words in a child's life. Books are an important resource in surrounding a child with language. Gorgeous illustrations, such as those in The Wild Swans (really, any of Susan Jeffers books) can help teach the meanings of the words in the story in a more natural way than having to stop and "teach" the new words. Don't forget that children absorb words over time, so one reading doesn't necessarily do it.
Another reason for reading more complex books is for the story structure. In Goodnight Moon, all that happens is a little bunny going to bed. In Cloudy with a Chance of Meatballs though, the sky is literally falling, along with pancakes, drifts of mashed potatoes, and all kinds of crazy fun. You'll have the chance to discuss much more with your child after reading this type of book.
Books can also help you teach your child about history or science. When you child sees you look up when to plant okra in your gardening book, s/he realizes that books are for more than stories. Share non-fiction with your child, looking for books with connected text, such as One Tiny Turtle as well as those that contain mostly illustrations with captions.
The final reason to read more complex text to your children is that it is more fun for you! I'm currently about halfway through Charlotte's Web with my daughters. I know they aren't getting it all, but they are thoroughly engaged. I read just one or two chapters a day and it's idyllic--we read all snuggled up together in my bed. These are the moments moms, frugal or not, live for!