Given the dimensions of the book: "two feet broad by three feet in height, and over four inches thick", and being its pages made of "polished plates of electrum" (data from Forgotten Realms Campaign Set - DM's Sourcebook of the Realms, p. 92; TSR, 1987), what would be the weight of the book?
Assuming the book is effectively a solid made of electrum 24 × 36 × 4 inches, its volume would be 3,456 cubic inches. This is equal to 56,634 cm³.
Given the density range above, this means it could weigh anything from 707 to 934 kg (1559 to 2059 lbs.). Based on our earlier assumption that the book is a solid, the actual weight would probably be a little less than that.
† The densities in the second source appears to have a minor magnitude error, in that 1,500 kg/m³ is impossibly light when electrum is composed of silver (~10,000 kg/m³) and gold (~20,000 kg/m³). If the digits are assumed to be correct, adjusting the magnitude to match that of silver and gold yields the sensible values of 15,000 kg/m³ and 16,500 kg/m³, which is 15g/cm³ and 16.5g/cm³.