T-shirts are widely used to convey love messages, or protest quotes, or affiliations, or anything that may be of interest to others. T-shirts are like billboards.
The T-shirt here is used to display an impossible task ... the task of reading from beginning to end The Book of Sand.
The impossibility consists in reading infinite book; no way to begin, no way to end.
To see where is the irony, see The curious infinite of Jorge Luis Borges.
