Betty Lou's Library won't be for everyone, but if you like feeling like you've stepped back in time to Prohibition 1920s, then you'll get a kick out of coming here. The cocktails are superb; if you can't decide just ask one of the bartenders for a custom-crafted cup of yum. You'll need a password to enter, so call first.
The entrance is actually on 16th Ave, not 17th. Look for a red light, a bookshelf and a phone.
Note also that there are some house rules that are enforced – among them, no talking or making chitchat with other parties at the bar. That means this spot is best enjoyed with a friend or two, unless you like drinking alone.