Print-based:

[1]Princeton Field Guides : Trees of Western North America, Nelson, Gil; Earle, Christopher J; Spellenberg, Richard

The resource of this book can also be found on the internet. You can also found the table of contents which is linked to different chapters on the website. This makes it quite easy to find what you need.

Besides the detailed introduction, what I like most is the quick ID which makes people quickly remember the characteristic of the tree. It also has the similar species part which help people to tell the differences between similar ones.

Digital

[1]http://conifersaroundtheworld.com/photos/thujopsis_dolabrata_cones_kórnik_arboretum_poland

I find this website very useful when I am learning the conifers. The name of this website is conifers around the world. Everything on this website is about conifers. It has some really excellent photos which can help you the characteristic of different conifers.

[2] http://www.botanical-online.com/botanical3.htm

This website is different from others. It is not about the information of different plants. It explains the proper names of botanical. While preparing the last exam, I was quite confused by the dry fruit. When I searched on the internet, I found this website which helped me understand what kind of fruit can be called dry fruit, and what characteristics they have.