Bookshelves don't have to be expensive solid oak ones. We couldn't afford to invest in major wall units or solid wood. Instead we went to Home Depot and perused their storage aisles. They have white laminate units (and wood tones too) in all different sizes ranging from $12 to $60 with adjustable shelves. We bought 5 of these in different sizes. We placed them in places where you wouldn't notice that they weren't top-notch quality (under countertops or in an alcove). They are filled with stuff anyway, and most people who come here are too busy looking at the rather interesting things we've collected instead of looking at the furniture. Another trick to hide the inexpensive look of them is to cover them with a table runner.
One of our units doubles as a sort of "sensory table" - like they have in preschools. Our nature collections, magnifying glasses, and related materials are displayed on the top of it. Magnets, a balance scale, cuisenaire rods, abecus, and globe are stored on the shelves. This is kept in the kitchen. Our kitchen door leads to the backyard, so it made sense to keep all of our science stuff in the kitchen. A bookshelf in the kitchen is also great when you need to transfer works-in-progress to somewhere else when they are taking over the kitchen table and it's time to eat dinner!