A riff is the same as a lick, a short melodic phrase or lead line. Usually a catchy series of notes that make people want to play "air" guitar.
Shuffle is a type of rhythm, or a way to play a rhythm. Similar to "swing" in jazz.
Each quarter note ( 1 beat ) usually breaks down into 2 eighth notes, counted : 1+ 2+ 3+ 4+
In a shuffle, each beat is treated as a triplet, counted 1+a 2+a 3+a 4+a. It will often be written as a straight 4/4, but the eighth notes will be played as a triplet with the first note lasting for 2 counts of the triplet, and the second on the last 1 count.
So the first will last for 1+, and the second will be on "a". (pronounced, "uh").Simply put, the first eighth will be long, and the second will be short.
When the actual rhythm is written precisely, it is written as being in 12/8 time. Most don't bother, and just write in 4/4 and tell you to play as a shuffle.
The words "shuffle" and "riff" are not related. They deal with completely different aspects of music. You cannot use them interchangeably and still make any sense at all.
When they group the two ideas in the book, it is because the style of music may be played as a shuffle, and certain riffs may be expected to work in that idiom.