[pxx]

books isn't a valid solution. it's on the guesslist but not the wordlist.

[mrb]

Correct. In fact, thousands of plural words are allowed as guesses, but the solution will never be plural. Source: I grep'd the lists of solutions and authorized guesses.

[eco]

I used the combined solution and valid word list when I was playing around with a program to find good starting words. Feels too much like cheating otherwise.