UNIX file reading

i have a directory that has n files starting with ABC_00x.txt and ABC_00x.txt where x is the integer number.
i want to read the contents of each file and write it into a log file.
can somebody help me in writing the shell script in unix

something like this will work...

for n in `ls`
do
cat $n >> out.log
done

If x is a single digit number.

cat ABC_00?.txt >> log.file

may be just?

cat ABC_00[0-9]*.txt > file.log