Reddit reviews Teach Yourself Latvian (Teach Yourself Complete Courses)

I was (still am) in a similar position to you, after a lot of reading about learning languages in general (including polyglot and polyglot ama threads on Reddit) I read and concluded that the "Teach Yourself X" series were the most effective (Rosetta is overrated and another popular one I forget the name of).

You might have to Google a bit, I just did a quick one and couldn't immediately see any for under £100 Amazon UK link.

Last point, I'd suggest that you simply use it as much as you can, (a common theme from the polyglots), so secrecy is probably not an option.