Piet Oudolf, Probably the most famous naturalistic garden designer and a great source of inspiration. 
Among all his great work, we highly recommend his book written in 2013 with Noel Kingsburry, Planting, a new perspective.
www.oudolf.com

Enjoy the video below .....

Dr Noel Kingsburry,  Author of many books. Noel has a great Blog and website where you will find lots of references and links for a much deeper understanding of perennials and naturalistic design. I also lately really enjoyed the lecture of his very original book, Garden Flora, The natural and cultural history of plants in your garden. 
​www.noelkingsbury.com, www.noels-garden.blogspot.ca

Thomas Rainer, Landscape architect from the US. In 2015 Thomas wrote a great book with Claudia West : Planting in a post- wild world, which shows an approach and philosophy similar to ours. An approach based on a meaningful and sustainable design as well as a reflection and Nature and Culture, Landscape and History. 
Here is the link to his blog : www.landscapeofmeaning.blogspot.ca

For a good methodology to learn and identify plants we recommend the book of Thomas J. Elpel. Botany in a day, the patterns method of plants identification. Hops Press

Travis Beck, Principal of ecological landscape designs. Excellent source of information and a reference book.