This pocket-sized guide is great for field identification and specializes in plants of north-eastern and north-central North America. Its key system usually gets you to the right plant and the line drawings are more useful than photographs. Because taxonomic fiddling is never ending, some of the Latin names have changed since the book was published in the 1970s, but this has been a reliable help for me on many hikes.
This excellent guide covers all native trees in Canada as well as exotic species that have spread into the landscape (e.g., Buckthorn, Weeping Willow, Norway Maple, etc.) There are also species that occupy that shadowy zone between shrub and tree depending on growing conditions (e.g., Sumac and Elderberry). Each species has a detailed entry with line drawings, photographs, descriptions, range maps and quick recognition features. Keys for groups of genera and winter twig identification are included.
Here is a thorough field guide for Ontario shrubs that includes large line drawings, range maps as well as general and genus keys. It's useful for those just out for a stroll doing casual plant identification as well as for those of us determined to pin down the exact number of willow species growing in a marsh. Each entry includes a "Field check" with a list of quick features to separate the plant from similar species. At just under 500 pages it's a bit big to carry in your pocket, but it fits easily in a knapsack.
Use this guide to identify invasive or non-native plants. The book has a strong agricultural emphasis, but it provides great information for what risks each species presents to native plant communities. Seedling plants are also pictured ( a great help when weeding!)