You really don't need to know much in order to use the Avenson Audio Small DI. Once you've made your connections you're good to go because it doesn't have any parameters or anything. Just make sure you've engaged phantom power on the other end since it does require it. A manual definitely isn't necessary, but I haven't seen one for this box so I can't speak to it's make up.
The sound of the Avenson Audio Small DI is everything you'd want in a DI box, as it won't color your sound at all and transparently relays information from your instrument to your preamp. The box can be used for recording anything where you'd want to grab a direct signal, but I've only used the box for recording bass guitar when I client brought it in. It doesn't really matter that there are no parameters since the goal is to get a clean, direct sound anyway...
I really don't have any complaints about the Avenson Audio Small DI, as it does a fine job for getting a DI signal. It wouldn't be my first choice for a DI since there are definitely models out there that are more versatile, but at the same time it would be adequate for me in almost any situation. The price isn't as cheap as you'd expect for a super small DI, but it's still not too bad. Those who are in the market for a nice sounding DI box sound definitely look into the Avenson Small DI.