The shorthand notebook that the question is referencing is Sydelle Pulaski's notebook.  She took notes in it in order to document what the will said the first time it was read through.  The notebook ends up being quite valuable, and Pulaski is aware that her notes give her team a distinct advantage.  In order to keep it safe, she hides it in her room. However, the notebook is stolen.  

Sydelle Pulaski stared at the bare wicker bottom, then sank to the rim of the bathtub, shaking her head in disbelief. Someone in Sunset Towers had stolen the shorthand notebook.

The notebook reappears in chapter 11.  Mr. Hoo finds the notebook on a table in his restaurant in the morning.  He is accessed of being the thief, but he explains how he came by the notebook. 

Her self-congratulatory pose was too much for Mr. Hoo. Grunting loudly, he squeezed out of the booth and slapped the shorthand pad on the counter.

“Thief!” the secretary shrieked, nearly toppling off the stool as she grabbed her notebook. “Thief!”

“I did not steal your notebook,” the indignant Hoo explained. “I found it on a table in my restaurant this morning."

Mr. Hoo goes on to explain that the notebook is completely useless to him anyway because he and his partner cannot read the shorthand.  Mrs. Wexler is capable of reading shorthand, but she believes that Pulaski's shorthand is "gibberish." 

“Pure jibberish,” Grace Wexler added. “Those are standard shorthand symbols all right, but they don’t translate into words.”

Pulaski goes on to explain that she was worried that she could not trust anyone. As a result, she wrote all of her shorthand notes in Polish. 

I’m no fool, you know. I knew I couldn’t trust any one of you. You can’t read my shorthand because I wrote in Polish.