I recorded my main audio book in stereo at a studio and want to re-record the intro and outro through audacity. Upon recording, it does not give me the ability to selected stereo channel. I only have the mono option.
Unless your microphone is stereo, there’s no point recording in stereo:
it will be dual-mono, where both tracks are absolutely identical, same as mono.
Odds-are your audio-book studio-recording will be (dual) mono too.
Note that a (normal) single mic or single voice/instrument is mono. Stereo normally requires separate left & right microphones (or a stereo mic) and different sounds on the left & right.
You can make a “dual mono” file with the same thing in left & right (“mono” CDs always have two identical channels) but a true-mono file will play through both speakers. There’s no advantage to dual-mono. It just makes the file twice the size (if uncompressed).