I seriously doubt C. S. Lewis would make a typo. This is indeed spoken language, but it is also standard in some dialects. Yale University has an article on Inversion in embedded questions, commenting on examples such as
And he told them who was it. (Fought 2003:98)
The article says
In the syntactic literature, this construction is discussed in some detail by McCloskey (1992) for Hiberno English and Henry (1995) for Belfast English. It has been reported in a number of North American English varieties as well, including African American English (Green 2002:87-89), Appalachian English (Wolfram and Christian 1976:129), Chicano English (Fought 2003:98), Northeast U.S. English (Ross 1975) and Newfoundland English (Clarke 2004:315). This construction has become rather widespread, according to the following passage from Wolfram and Schilling (2016:388):
[...] the use of inverted word order in indirect questions, as in
- She asked could she go to the movies,
is becoming just as much a part of informal spoken American English as indirect questions without inverted word order, as in
- She asked if she could go to the movies.
Quoting, Shane Walshe, Irish English as Represented in Film, 2009,Thought.co also signals dialectal usage:
Some dialects of English (including Irish English and Welsh English) retain the inversion of direct questions, resulting in sentences such as
I asked him was he going home.
Now, C. S. Lewis was born in Belfast, so it's not surprising he uses this structure.