When do you use von and when do you use a double-nominative construction, such as discussed at this linked question, when talking about the amount of something? Compare, for example,
ein Glas Wasser
tausende von Büchern
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The von construction can be used with indefinite number adjectives, such as but not limited to "dutzende, hunderte, tausende, ...". But it is optional.
Thus, both sentences are grammatically correct and used:
Es wurden tausende Bücher verbrannt.
Es wurden tausende von Büchern verbrannt.
The latter construction puts more emphasis on the number.
"Ich möchte ein Glas von Wasser" would be ungrammatical, because here you have a definite amount of glasses, namely 1.
This is a Partitive Apposition, see Canoo.net. It is just specification of an amount. Similarly constructed are drei Liter Wasser, 100g Mehl.