Two good resources for learning about plant
roots, rooting depths, and related topics: David Jacke's
Edible Forest Gardens has a chapter that has many diagrams of root patterns and a detailed discussion of how to use them in design; and Robert Kourik's
Roots Demystified: Change Your Garden Habits to Help Roots Thrive which has many diagrams of roots and a lot of information on how to create non-competing interplant arrangements. There's also an older book called
High-Yield Gardening by Marjorie Hunt that has many good non-competing polycultures in it with notes about differences in root depth and competition.