Google is your best bet, here's where you'll learn the variants:
VARIANTS (Model/manufacturer)
• SVD, Russia
• SVDS, Russia
• SVDK, Russia
• SVU, Russia
• FPK (PSL), Romania (My note: not a Dragunov from an engineering perspective)
• Type 79 (DM-86), China
• Al-Kadesih, Iraq
As for books, unless you have access to Russian books and can read Russian, all I can readily suggest are pages 115 to 119 that cover the development of the SVD beginning in 1958, and ends with a brief description of the revised 1994 era SVDS. Has about 6 of 7 pictures as well.
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The SVD was developed from 1958 to 1963. It was eventually updated in 1994.
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7N1 is the ammo mentioned at top of this except:
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FWIW, my understanding is the last 100% Russian-made 100 SVDs were imported into the USA in 1994…hence their very high prices.