I’m not familiar with either of these books, but the second is closer to current reality. The first may be discussing a specific processor.
Processors have been made with quite a variety of word sizes, not always a multiple of 8.
The 8086 and 8087 processors used 16 bit words, and it’s likely this is the machine the first author was writing about.
More recent processors commonly use 32 or 64 bit words.
In the 50’s and 60’s there were machines with words sizes that seem quite strange to us now, such as 4, 9 and 36. Since about the 70’s word size has commonly been a power of 2 and a multiple of 8.