I think I can actually figure out what your book is doing, Phucuong.
First let's figure out what the "7" in "major 7" means. Supposing that you have a chord where the root (lowest note) is C. All of the notes starting from C are C,D,E,F,G,A,B,C. If you count them, C is note no. 1, B is note no. 2 etc. So in CEG we are using notes 1,3,5. If we skip another two notes we get to note number 7, which is B. So CEGB is some kind of 7 chord. There are two possible kinds of 7 chords. CEGB has a "major 7" chord because of how high up the 7 is. If it were any higher, like a half tone higher, it would be another C. That's as high as it can go. Another 7 chord would be CEGBb. that 7 chord is lower down. It is simply called a 7 chord.
Chords can be in any order. Like the "C chord" can be CEG, GCE, ECG and it's still the C chord. Same for your 7 chords.
For what your book is doing (I think): So supposing that you start with CEG. If you play a note just under that C by going to the next key to the left, then you get BCEG. That has all the notes of our major chord. It is CEGB but spelled differently. If instead you play a note that is two half steps or two piano keys down from the C you get BbCEG, which is like the CEGBb spelled differently. So what they are doing "works" and it's easy to do on the piano. It just makes it hard to understand what these 7 chords actually are.
I think the diagram in the link explains it much better.