Confused between the terms Process and Process Control Block
The point of swapping is to make room in RAM. What is moved to disk is what the book calls “process in memory”, not the process control block. The PCB block is a small amount of memory, so moving it to disk wouldn't help much. Additionally, some of the PCB is necessary even if the process is never scheduled, for example to know whether a file is open or not (which matters if the file is deleted or when taking a lock on the file), to know what events (such as I/O becoming available) might cause the process to become runnable (this information would be in the “process state”), and the knowledge of where the process data is in the swap space (that would be the process's “memory limits” while it's swapped out). In contrast, none of the process's own memory is necessary while the process is not scheduled.
Note that most operating systems don't swap out a whole process, but parts of its memory. Some books make a distinction between “swapping” a whole process and “paging” parts of memory, but in modern usage, the words “swapping” and “paging” are usually synonymous and refer to any mechanism that temporarily moves process memory to swap, whether it moves a whole process at a time (uncommon) or parts of process memory (common).