It’s a closed-stack library and their catalog is online. Figure out the items you want to start with before you arrive so that you can put in the order; it takes a little while for the staff to fetch it from the stacks and deliver it to the reading room.

Also, pick your reading room carefully: the general collection can be delivered anywhere, but there are are some items that you can only get in a specific one.

It may be the only way to find some of thing things preserved via citizen archives. I enjoyed the talk Ian MacKaye gave there about archiving the live performances of Minor Threat & Fugazi. Here [1] is a post on the subject.

[1] https://blogs.loc.gov/thesignal/2013/05/ian-mackaye-and-citi...

Be sure to visit the Jefferson Building, its a "temple to learning" with wonderful paintings and tiles.