My native language is not English. This paragraph seems too complicated for me. What did "miss crail" told her mother? The whole paragraph is confusing
I have to make some guesses about what has gone on before.
Evidently there is a man (Leamas) who has come to the library very regularly, probably every day. This paragraph starts off by saying there was a day he did not come, and that the fact that he did not come "delighted" Miss Crail for some reason not mentioned here. She told her mother, and after lunch was staring at the shelves where this man had been doing things in the library on the days he was there. She evidently wants people to think that she is trying to figure out if the man had stolen any of the books, the paragraph implies that she is more interested in having people think she is doing that than actually doing that -- that's what I get out of the "theatrical" description and out of the "pretending" part of the description.