。
I've got a ton of books.
Literal
Book [object-を] tons hold-[ongoing-ている].
しこたま is a colloquial adverb meaning 'a huge amount, tons.' It has a folksy, casual feel — rougher than たくさん and with a hint of exasperation or emphasis. 持っている ('own, have,' literally 'am holding') uses ている for the stative 'own' sense typical of possession verbs.