Suggestion.

https://www.google.com.br/search?q="most easily observed spe...

http://www.adventurepublications.net/catalogsearch/result/?q...

Portuguese term (edited): 50 espécies mais fáceis de serem observadas

I think this is a reasonable equivalent, so "xxx: a field guide to the 50 most common species."

e.g.
Most commonly found along water courses such as the Rio Grande or where flowers are abundant after rain, they can be found somewhere at any time in the vast, varied land west of the Pecos. This book describes and illustrates the ***** fifty most common **** butterflies to be found in the region, along with eleven additional specialties unique to the far western part of the state.
http://www.abebooks.com/servlet/BrowseListingsResults?n=1002...

Egbert Leigh, of STRI, has arranged for artist George Angehr and Malagasy botanist M. Abraham, of the Service deu Eaux et Forets, to prepare an illustrated guide to the **** fifty most common species *** of trees in a forest reserve near Perinet between Tananarive and the east coast of Madagascar.
http://archive.org/stream/smithsonianyeara8587smit/smithsoni...

Nice Book, Covers Some of the Most Common Species of Birds in North America
http://www.amazon.com/Birds-Spiritual-Symbology-Significance...

Each Falcon Field Guide to birds introduces the 180 most common and sought-after species in a state
http://www.giftsforeveryone2.com/category/gifts-for-the-bird...