。
He has twice as many books as I do.
Literal
He [topic-は] my 2-times [possessive-の] books [object-を] has.
~倍 ('-fold, times') is the standard Japanese suffix for multiplicative comparison. The structure 私の2倍の本 ('books 2 times mine') uses two layers of の: one to attach 倍 to a quantity, and one to link the comparative phrase to the noun. Note this sentence breaks the 彼女 pattern of the surrounding sentences — 彼 (he) instead. Counting comparisons like this are formed compactly in Japanese: just [base] の + [N倍] + の + [thing], without the 'as much as' English requires.