My favorite explanations for this moment, in order of how much they make me laugh:
Likely Doylist explanation: Bram Stoker wants you to realize this is weird and feel nervous about the Count because of it.
Likely Watsonian explanation: This is another facet of Dracula’s studying, so that he’ll blend in better once he gets to England.
Unlikely but funny Doylist explanation: Bram Stoker (as will be shown later) thinks this is a reasonable thing to do, and the sign of a species of Superior Intellect.
Funnier Watsonian explanation: While Jonathan was fetching paperwork, Dracula ran into the other room, set the table at superspeed, ran back to the library, grabbed the first book his hand fell on, and carefully posed himself to look like he’d been there the whole time. Whereupon he realized that he had picked up the Weirdest Possible Casual Reading Choice but didn’t have time to pick another book and now has to brazen it out. Like a cat, Dracula at all times has to act like He Meant to Do That. He’s studying train timetables - of course he is! That’s a completely reasonable and normal human thing to do in preparation for moving to a new country! Train travel is a vital piece of British infrastructure that he needs to understand!
Jonathan now thinks that Dracula is a train enthusiast.