drink it all in ?

It means to witness/absorb/experience or otherwise accept some sort of sensory input involving whatever the target 'it' is. Like accepting all of the available information or basking in it.

Best I could do this late.

Yes, to drink it all in means to take up all the aspects of the thing named, with all the applicable senses. So, for example, if you visited a farm, and you drank it all in, you would see everything you could in as much detail as possible, and you'd smell the smells and hear the sounds, experiencing as much as possible.

By the way, the opening sentence is in error: I need to observe more of Lucinda and Daniel, drink it all in until something began to make sense.

Need is present tense, while began is past tense. It should either read "I needed to observe . . . " or it should read " . . . until something begins to make sense."