I just knitted mittens on single needles and my question is:

How do I knit them together?
When I tried to sew or knit them the seam looks awful -
it’s not a smooth closure. Are there any books on this that your aware of?

Feel so sillyasking this but I just think it looks bad-

See if the finishing section of videos helps - in particular matress stitch.

You can also try a search at You Tube for ‘knitting seams’ or similar for a numerous number of videos.