Shannon did some experiments to determine the entropy in English texts. A later work done by Cover and King  gave an estimate of 1.34 bits per letter. This implies that, if the letters are coded into 5 bits, one needs to appropriately combine 4 text files in order to obtain bit sequences of full entropy, since 4*1.34 = 5.36 > 5. The method used in our software is to sum (mod 32) the coded values of a-z (mapped to 0-25) as 5 bits of the corresponding letters of the text files.
There are plenty of other schemes for obtaining high quality pseudo-random sequences in practice, e.g. AES in counter mode. However our scheme seems to be much simpler both in the underlying logic (understandability) and in implementation and is thus a viable alternative that one could use/need under circumstances.