。
Reading 10 books a day — he must be a bookworm.
Literal
Every-day 10 volumes [possessive-の] books [object-を] read [surprise-とは] he [topic-は] book [possessive-の] bug [certainly-に違いない].
とは after the clause expresses surprise or disbelief at the preceding fact. 本の虫 ('book bug') is a calque of the English 'bookworm.' ~に違いない ('there's no doubt that') expresses strong conviction.