"Blue covered Nancy Drews:" The blue-covered Nancy Drews were published between 1930-1961. They are pretty hard to find. I have two of them. I wish I could find more!
The Ringmaster's Secret 1953
The Mystery of the Ivory Charm 1936
"Yellow Nancy Drews:" Beginning in 1962, the Nancy Drews did not come with a dust jacket, and the spines are yellow. These are easy to spot and can be found at yard sales, junk shops, antique stores, etc.
The Secret of the Old Clock (1) 1959
The Mystery at Lilac Inn (4) 1987
The Clue in the Diary (7) 1987
The Sign of the Twisted Candles (9) 1968
Password to Larkspur Lane (10) 1966
The Mystery of the Ivory Charm (13)- 2 copies: 1936, 1974
The Clue in the Jewel Box (20) 1972
The Clue in the Crumbling Wall (22) 1973
Mystery of the Tolling Bell (23) 1973
The Clue in the Old Album (24) 1947
The Clue of the Leaning Chimney (26) 1949
The Secret of the Wooden Lady (27) 1967
The Moonstone Castle Mystery (40) 1963
The Mystery of the 99 Steps (43) 1966
The Secret of Mirror Bay (49) 1972
The Secret of the Forgotten City (52) 1975
The Sky Phantom (53) 1976
The Thirteenth Pearl (56) 1979
My favorite Nancy Drew book is #13, The Mystery of the Ivory Charm. I'm not sure why- I've just always loved it! I have three copies that James bought on eBay for me a few years ago. Two yellow spines and the coveted 1936 blue-tweed cover.