This book outlines which veggies (and sometimes fruit) to plant together. This is called companion planting. The idea is that different types of plants in close proximity can benefit each other. For example, planting your carrots near onions and leeks is beneficial for the carrots since the onions and leeks act as a repellent to the carrot fly.
A gardener can also gain a higher yield per acre by integrating plantings instead of abiding to a mono-cultural method (i.e. rows and rows of carrots, followed by rows and rows of beets). Planting rows which contain a kohlrabi - beet - kohlrabi pattern benefits both crops. They both do best in the same type of soil and pull their nutrients from different levels. Anytime you diversify your crops, you make it harder for critters to take out your entire patch.
If there is a worm that just loves cucumbers, he and his buddies can easily demolish one plant, but if the next plant is a distance away from the one he destroyed, he must figure out how to get over to the new cucumber plant. Chances are he won't make it the entire way, and your other plant is safe. Or you have time to take protective measures. But if all of your cucumber plants are close and hanging out together, it will be that much harder to save your plants.
The book also discusses which plants are detrimental to each other. As well as the different ways to cope with certain diseases and pests and basic descriptions of each plant. While this book is not exhaustive (and therefore NOT overwhelming), the array of plants it covers varies from vegetable and herbs to nuts and wild plants. There are also chapters on gardening techniques, garden plans and pest control.
This book is a wonderful resource that I have by my side when I'm planning my garden. Although of this information is easily accessible via the internet, having a small, easy to read book (that won't break if it get's wet or dirty) by your side makes life a lot easier. I would recommend this book and have used it often and will continue to use it over the years.