$ cat data1.txt
A:list1,list2,list3,list4
B:list1,list2,list3,list4
C:list1,list2,list6
D:list3
F:list2,list4,list5
G:list7
H:list2,list5
A:list1,list2,list3,list4
B:list1,list2,list3,list4
C:list1,list2,list6
D:list3
F:list2,list4,list5
G:list7
H:list2,list5
 
$ cat data2.txt
list1:A,B,C
list2:A,B,C,F,H
list3:A,B,D
list4:A,B,F
list5:H,F
list6:C
list7:G
 
desired output:
A:B,C,D,F,H
B:A,C,D,F,H
C:A,B,F,H
D:A,B
F:A,B,C,H
H:A,B,C,F

#!/bin/awk -f
 
BEGIN { FS=":"; ts1 = "date \"+%s\""; }
NR == FNR { bookLists[$1]=$2; next }    #the first file
{ books[$1]=$2 }                                        #the second file
END {
{ print "There are in total", length(bookLists), "book lists and ", length(books), "books." > "make_data_log.txt" }
count = 0
#bookLists: A:list1,list2
#books: list1:A,B
cmd = "sort -k 3 -nr | awk ' { arr[$2]; if(!b) b=$1 } END { for (i in arr) str=str ? str\",\"i : i; print b, str} '"
for (i in bookLists) {
        #print "========== book " i "=========="
        split(bookLists[i], tmpBls, /,/)
        for (j in tmpBls) {
                split(books[tmpBls[j]], tmpBs, /,/)
                for (k in tmpBs)
                        ++result[tmpBs[k]]      #print i":", tmpBs[k]
                delete tmpBs
        }
        for (l in result) {
                if (i != l) print i, l, result[l] | cmd
                #if (i != l) print i, l, result[l]
                if (++num == 20) break
        }
        close(cmd)              #close the pipe, or this "sort" will be delayed until the awk program ends
        delete result
        delete tmpBls
        num = 0
        if (++count % 100000 == 0)
                print count >> "make_data_log.txt"
}
 
{ ts2 = "date \"+%s\""; print ts2-ts1, "seconds consumed." >> "make_data_log.txt" }
 
}