This app was mentioned in 7 comments, with an average of 2.00 upvotes
Just an FYI, the church has created Apple and Android Book of Mormon apps, that are specifically for these kinds of situations. They have in-app ways of asking questions about doctrine that are answered by operators online - all in the app. It's a lot less overwhelming for an investigator than Gospel Library.
The easiest way would be to read it online:
https://www.lds.org/scriptures?lang=spa
There is also an app for just the Book of Mormon in Spanish put out by the church. The link below is for Android but they also have it for iOS, Windows Phone, BlackBerry, and Kindle. Just use the link above to find the other links.
https://play.google.com/store/apps/details?id=org.lds.bom&hl=es
They are using electronic notes to make sure they don't forget to follow up with you. While it seems a little odd for them to send you this, it's pretty standard for organizations to use your data in this way.
FYI if you want to read the Book of Mormon on an electronic device. There's an app for that or you can get the ebook for free on Amazon. But honestly, if would be good to have the missionaries around to explain stuff every now and then.
Lue: https://play.google.com/store/apps/details?id=org.lds.bom (Itse luen ihan vain mielenkiinnosta. En ole käännytystyössä)
or even just book of mormon. it is available as a standalone app.
https://play.google.com/store/apps/details?id=org.lds.bom&hl=en_US
https://apps.apple.com/us/app/the-book-of-mormon-another-testament-of-jesus-christ/id547313550
You can download it for free on the Google Play Store (Probably Apple too, I just don't use it).
It's not meant to be a "bootleg" or "fanfic," more like a companion book or another book in the same timeframe/world/topic. Pretty good read, even if you have no intention of converting :-P