Here is a list of good websites to browse through! These are websites that we've used over the years to learn about the plants we grow, as well as general gardening tips.
This website, published by the USDA forestry service, is probably one of the most comprehensive publications on native conifers on the web. You'll learn everything from native range and soil types a species grows on, to physiological characteristics and life history! These articles are a bit scientific to read, but if you're curious about native plants, they're a good resource.
A companion publication to the above, this section of the same website covers native hardwoods. This is probably one of the most helpful resources to begin understanding why certain trees grow where they do. You'll learn what pests and diseases certain species are prone to, and growth habits of young saplings. While the publication has a commercial forestry bent, you'll find the information helpful for landscaping and gardening, as well.
A well-researched and observational account of many native wildflowers, this website is an excellent, easy-to-read and well organized resource for Midwest native plant gardeners. There are detailed accounts of many of the woodland and prairie wildflowers native to Illinois, though the information applies to much of the Midwest.