I own 150 books, but I have no bookcase. Because nobody will lend me a bookcase.

The line is built on a comic inversion of expectations: books are commonly borrowed, while furniture is not. By claiming he owns many books yet lacks a bookcase because “nobody will lend” one, Youngman turns a mundane domestic problem into a joke about social norms and stinginess. The humor also plays on the implied absurdity of a person who can accumulate books but cannot manage the basic infrastructure to store them—suggesting a self-deprecating persona of mild incompetence or perpetual misfortune. Like much of Youngman’s one-liner style, it’s less a literal complaint than a tightly structured gag that pivots on the unexpected object of lending.