I think your library will reflect your interests.
I started with Peterson and I still go back to it for basics. My 1980 copy (birds) is a little out of date now, but I find the field mark highlights and the maps are useful.
Peterson is also my basic wildflower book. It classifies flowers on that most significant of markers, flower colour. I have a Newcomb too, but there's an amazing investment in learning the organization of a book, and I don't use it much. (It doesn't help that I often miscount the number of petals in a flower.) The disadvantage of wildflower guides is just that--they're for the flowers, & are not much help for the greater part of the year when plants are not in flower.
For trees I like Linda Kershaw's Trees of Ontario, published by Lone Pine. I like the leaf key in particular. (I hardly ever leave the province, so an Ontario guide is fine for me.) Other favourites are emphasize habitat, Lone Pine again, Wetland Plants of Ontario and Forest Plants of Central Ontario, both with multiple authors. (I really, really like the wetland plants book, especially from the bow of a canoe. It's amazing what you can see from the water. Stay off Lake Ontario, however, unless you really know what you're doing.) Peterson has a new guide to Eastern Forests, emphasizing ecology rather than species identification. It's worth a read, but it's very US-biased, & a little lax in places on keeping northern & southern ecosystems separate, so not all the phenomena apply here.
My new treat this fall is Soper & Heimburger's Shrubs of Ontario, published by the ROM. My local bookseller was able to order it for me.
Living on the shelves is The Atlas of the Breeding Birds of Ontario, 1987 version, getting a little long in the tooth.
Now that I have a DSLR with a 70-300mm zoom lens (and a really bad case of lens envy, but that's another story), I use Cornell's allaboutbirds site to help in identification. And I haven't found a butterfly guide to beat Walter's display on this site.
