。
She has twice as many books as he does.
Literal
She [topic-は] his [genitive-の] two-times [genitive-の] book [object-を] is-having.
The neutral, informational form of the multiplier comparison: 2倍の本 ('twice as many books'), with の linking the multiplier (2倍) to the noun (本) attributively. The emphatic counterpart would be 2倍も本を持っている (with も marking 'as much as twice'); without も, this version simply states the quantity without surprise. Note Arabic numerals (2) here rather than the kanji 二 — both are common in modern Japanese, with kanji feeling more traditional.