Hellobee Boards

I think in this case it's more a question of quantity over quality. Just go to the library weekly and make sure you are not getting the same author every time. Your LO will be exposed to way more words that way than from any single book.

This resource had some good information: http://www.pbs.org/parents/education/reading-language/reading-milestones/toddler-language-development-milestones/toddler-reading/

For language development talking and reading are so important. Re-reading books is actually a great thing: it helps foster the connection between the words and the pictures, helping children develop new language. http://www.heatherhaupt.com/2014/06/16/why-kids-need-us-to-read-the-same-book-over-and-over-again/
I just always try to make sure reading is fun. Books don't have to be award winners, or have a certain formula. The most important thing is to interact. Go ahead and read the same book over and over again. If he loves Eric Carle and is really drawn to the animals, seek out some more animal focused books. You can sure try some alphabet books (there's a Hidden Minnesota Alphabet one and a Brian Wildsmith one that were both hits at our house), or some books about shapes and colors, but you can use almost any book if he has some interest. Point out colors as you see them ("Oh, look at this red bird"), and respond to his vocalizations. Or if you see a shape on a page you can point that out. I wouldn't stress too much over specifically working on those concepts at that age. Kids who are curious pick it up pretty organically.

This one is great: http://www.amazon.com/My-Little-Word-Book-Books/dp/0312493878

LO loves it and has learned so many words from it

This one was great!