If you live in the Eastern US, especially the South East, Virginia Tech and Auburn University both have VERY hood online resources for tree and plant identification.
They denote poisonous plants, but you will have to search that specific plant for details of how toxic and what the effects are... magic mushrooms, poison ivy and camphor are all denoted "poisonous" but their effects and danger are very different from each other.
Print books published but the same and also the Audubon society are the best to start with for field guides.
