Species ID books with pictures, or photos, will generally never be comprehensive enough to include all species, although with some there will be additional information it the text that will give details about other related species, varieties, hybrids or cultivars.
Importantly look for ID books that include an ID key, generally at the start, and learn how to use it. They are usually accompanied with a good explanation of how it works.
The key takes the form of
Does it have this...?
If it does go to 
If it doesn't go to 
And so on, making successive distinctions until you end up with the species.
They can take some getting the hang of, and you need to know the botanical terms that they refer to.
Also importantly, if you don't already have and use one, you must have a decent hand lense, or magnifying loupe, that you take with you everywhere. Often the deciding features can only be properly be assessed when looking close up, and you need to get to know what the species you are familiar with look like under the lense.
An experienced ecologist friend, an expert in flora and fungi, has a couple of books that have no images in at all, it is just the species ID key, that he takes with him when he is surveying. Granted he already knows his species very well, but you can't remember everything all the time and so you need a book that can tell you the final distinguishing features, even if it is text.
But again a hand lense is a good place to start. My favourite is an Eschenbach 10/40 Pocket hand-held magnifier from Germany http://www.eschenbach.com/cdd40611-ceda-4188-a5c3-882565859296/products-hand-held-magnifiers-pocket-detail.htm
Someone gave this site http://dendro.cnre.vt.edu/dendrology/syllabus/key/key1.cfm?state=&zone= as a good resource for trees in the US