This sounds very interesting and I'm looking forward to reading more.
How much time did you spend researching potential app categories?
Was your category decision driven by any hard data, other than anecdotal knowledge that Spanish bibles were a category that was selling and had search volume?
No hard data. I saw that Spanish Bibles were ranking in the Reference and Book categories, like you said.
Also important (but not "hard data") is that when I searched "la biblia" relatively few apps appeared in the search results compared to other search phrases.
How much time did you spend researching potential app categories?
Was your category decision driven by any hard data, other than anecdotal knowledge that Spanish bibles were a category that was selling and had search volume?