、。
One book is thin and the other is thick.
Literal
One-side's book [topic-は] thin, other-side [topic-は] thick.
片方...他方 is a paired set for 'one...the other' when comparing two items. The い-adjective 薄い takes its く form (薄く) to chain with the next clause — this connective form functions like 'and' between parallel descriptions. The contrastive は appears on both items being compared.