The Easton Press published leather bound copies of field guides.  Although they are not all written by Roger Tory Peterson, they were sponsored by the National Audubon Society and the Roger Tory Peterson Institute.  Each volume was issued with an “Ex Libris” bookplate. Each bookplate has a drawing by Roger Tory Peterson selected especially for that edition.  

If you wanted to have copies of all titles and all editions you would have:
  
50 titles + 8 titles with 2 editions = 58 volumes

Here are the 50 titles that were published in the Roger Tory Peterson Field Guide Series:

Advanced Birding   
Animal Tracks   
Atlantic Coast Fishes   
Atlantic Seashore   
Atlantic Shells  
Atmosphere   
Beetles   
Birds of Britain & Europe   
Birds of Eastern US, Signed  
Birds of Texas   
Birds of the American West   
Birds of the Caribbean    
Coral Reefs   
Eastern Birds' Nests 
Eastern Butterflies 
Eastern Forests 
Eastern Reptiles & Amphibians 
Eastern Trees 
Edible Wild Plants 
Ferns 
Freshwater Fishes 
Geology  
Hawks 
Hummingbirds  
Insects 
Mammals of Britain & Europe 
Mammals of North America 
Medicinal Plants 
Mexican Birds 
Moths 
Mushrooms 
Northeastern Wildflowers 
Northwestern Forests  
Pacific Coast Fishes 
Pacific Shells 
Pacific States Wildflowers 
Rocks & Minerals 
Rocky Mountain Wildflowers 
Southeastern & Caribbean Seashore 
Southwest & Texas Wildflowers  
Southwestern Forests  
Stars and Planets
Trees & Shrubs 
Venomous Animals & Poisonous Plants  
Warblers  
Western Birds' Nests 
Western Butterflies 
Western Forests  
Western Reptiles & Amphibians 
Western Trees 

In all, 42 of the titles were designated as part of the Fiftieth Anniversary Editions.  Here are their titles and dates of publication:

Advanced Birding (1990)   
Animal Tracks  (1985) 
Atlantic Coast Fishes (1986)   
Atlantic Seashore (1985)  
Atlantic Shells  (1986)   
Atmosphere (1985)   
Beetles  (1985)   
Birds of Britain & Europe, 4th Edition (1985)   
Birds of Eastern US, Signed (1984)   
Birds of Texas  (1985)   
Birds of the American West, 2nd Edition (1984)   
Coral Reefs (1985)   
Eastern Birds' Nests (1984) 
Eastern Butterflies (Klots, 1986) 
Eastern Forests (1988) 
Eastern Reptiles & Amphibians, 2nd Edition  (1984) 
Eastern Trees (1988) 
Edible Wild Plants (1985) 
Ferns (1985) 
Freshwater Fishes (1991) 
Hawks (1987) 
Insects (1984) 
Mammals of Britain & Europe (1986) 
Mammals of North America (1986) 
Medicinal Plants (1990) 
Mexican Birds  (1984) 
Moths (1984) 
Mushrooms (1987) 
Northeastern Wildflowers (1984) 
Pacific Coast Fishes (1984) 
Pacific Shells (1986) 
Pacific States Wildflowers (1984) 
Rocks & Minerals (1985) 
Rocky Mountain Wildflowers  (1985) 
Southeastern & Caribbean Seashore (1988) 
Southwest & Texas Wildflowers (1984) 
Stars and Planets, 2nd Edition (1984) 
Trees & Shrubs (1986) 
Western Birds' Nests (1986) 
Western Butterflies (1986) 
Western Reptiles & Amphibians (1985) 
Western Trees (1992)  

Here are the 8 titles that were never published as 50th Anniversary Editions and their dates of publication:

Birds of the Caribbean (1995)
Geology (1996)
Hummingbirds (2001)
Northwestern Forests (1999)
Southwestern Forests (1999)
Venomous Animals & Poisonous Plants (1994)
Warblers (1997)
Western Forests (1993)

Most of the titles that were published as both 50th Anniversary and non-50th Anniversary were published in the same year (in some cases the exact same volume from the same series was published in 2 different years).  Even if the two series were published in different years, only the 8 titles below differ in any significant way from one another.  It is unclear if the later editions were published as 50th Anniversary editions or not.

Birds of Britain & Europe, 4th Edition (1985) & 5th Edition (1993)  
Birds of the American West, 2nd Edition (1984) & 3rd Edition (1990)  
Eastern Butterflies (Klots, 1986) & (Opler, 1995, 2001)
Eastern Reptiles & Amphibians, 2nd Edition (1984)  & 3rd Edition (1991)                                 Eastern Forests,  1988 (368 pgs.) & 1998 (488 pgs.)                                                                   Rocks and Minerals, 4th Edition (1985, 317 pgs.) & 5th Edition (1985, 396 pgs.)                         Stars and Planets, 2nd Edition (1984) & 3rd Edition (1996)
Western Butterflies (Tilden & Smith, 1986) & 2nd Edition (Opler, 2001)