ART OF THE STEPPES by K. Jettmar, 1967. A good first book to own on the subject, lots of line drawings and pictures. You may have to get this from a used book dealer. I recommend using the bibliofind search form to search many dealers at once for the best price.
THE FIRST HORSEMEN by Frank Trippett and Time-Life Editors 1974. Lots of photos and information on the Scythian lifestyle and history. Probably the easiest book to find at your local library.
THE WORLD OF THE SCYTHIANS by Renate Rolle 1989. Photos and line drawings, not an archive of Scythian art but a good readable book on their history and culture. Great color drawings of decorated horse gear, saddles etc.
THE SARMATIANS by T. Sulimerski 1970. Definitive book on the "Eastern Scythian" tribes. Many line drawings of artifacts, some animal style.
THE ANIMAL STYLE IN SOUTH RUSSIA AND CHINA by Rostovtzeff 1929. By one of the founding researchers in the field. Many plates of artifacts.
SPLENDORS OF SCYTHIAN ART: TREASURES FROM SCYTHIAN TOMBS by Artamonov 1969. This is a fantastic book of photos of Scythian artifacts - many of them gold. Very thorough with lots of close-up shots. An expensive book, but if you are interested in the field it is worth every penny.
TRADERS AND RAIDERS ON CHINA'S NORTHERN FRONTIER by Jenny F. So and Emma C. Bunker 1995. This is a recent soft back book documenting many of the more interesting artifacts found in the eastern areas of barbarian domination up to border of China. Lots of animal art reference material.
ANCIENT BRONZES OF THE EASTERN EURASIAN STEPPES by Emma C. Bunker 1997. Catalogue of bronze animal style artifacts of the eastern tribes.
THE GOLDEN HOARD OF BACTRIA by Sarianidi 1985. Loaded with color photos of Kushan gold jewelry, weapons etc. Much gold with turquoise inlay, lots of close-up shots, a fascinating blend of Scythian, classical and Asian art.
THE CELTS by Gerhard Herm 1975. See pages 81, 106, 107.
THE ART OF THE CELTS by Lloyd and Jennifer Laing 1992. See pages 34, 50-51.
CELTIC ART: THE METHODS OF CONSTRUCTION by George Bain 1951, 73. See page 72.
THE ART OF ANCIENT IRAN by Roman Ghirsham. Many fine photos of Scythian and Cimmerian art from the Mesopotamian area.
THE ARTS OF ASSYRIA by Andre Parrot. Many fine pieces from Luristan illustrated.
(The above two books would likely be easiest to find at college or university library.)
CELTIC ART: THE METHODS OF CONSTRUCTION by George Bain 1951, 73. Page 86 - Connections between early British art and that of Phoenician and other eastern cultures. Page 101, 128 - For Celtic connections with Assyrian, Chinese, Persian and Turkistan arts.
"A Mummy Unearthed from the Pastures of Heaven" by Natalya Polosmak, National Geographic, October 1994. Article about the now famous "Ice Princess" excavated from a kurgan in southern Siberia.
"Guardians of the Gold" by Adrienne Mayor, Archaeology, Nov - Dec. 1994. Fascinating article speculating that the ancient Scythian (Saka) tales of griffins guarding gold may have had something to do with dinosaurs.
"Sarmatian Treasures of South Russia" by Mikhail Treister. Archaeology, Jan - Feb. 1997. Recent finds in gold and silver.
"Mummies of the Tarim Basin" by Victor Mair. Archaeology, Mar. - April 1993. Caucasoid mummies found in the Tarim Basin, China, show links with Hallstatt culture in Europe, and with Tocharians. Color picture of a seventh century A.D. wall painting of red and blonde-haired Tocharian knights.
"The Silk Road's Lost World" by Thomas Allen. National Geographic, Mar. 1996. Caucasian mummies and other finds of the Tarim Basin.
"The Golden Hoard of Bactria" by Victor Sarianidi, National Geographic, March 1990. Good place to take a look at the Kushan treasure, but to get the full effect of the Scythian influence see book by the same title and author above.