From New York
From Penn Station, take NJTransit Northeast Corridor line toward Trenton. Get off at Princeton Junction and transfer to the Dinky to Princeton, which takes about 5 minutes to get to campus.
Take SEPTA's Trenton Line Regional Rail (R7) to Trenton. Transfer to NJTransit Northeast Corridor train toward New York. Get off at Princeton Junction and transfer to the Dinky to Princeton, which takes about 5 minutes to get to campus.
From the Dinky station, it is approximately a 10 minute walk east across campus to Lewis Library (see attached map).
See also http://www.princeton.edu/main/visiting/travel/driving/.
From New York/New Brunswick
From New York, take the turnpike south to exit 9. Follow signs forRoute 18N briefly until you are able to exit onto Route 1S toward Trenton. Once on Route 1, travel 15-20 miles south of New Brunswick until you reach Washington Road (past Scudder's Mill Road but before Quakerbridge Road). Turn right onto Washington Road, and travel about a mile. Just past the lake, turn right onto Faculty Road. Turn left into parking lot 21 just after the gym.
Take 95N to exit 67 onto Route 1N. Travel about 3 miles on Route 1 to Washington Road, where there are two closely spaced stoplights. You need to turn left onto Washington Road, but the turning is a jughandle, so you must stay to the right and cross Route 1 at the light (follow signs for Princeton). After about a mile on Washington Road, turn right onto Faculty Road just after the lake. Turn left into parking lot 21 just after
The best place to park is lot 21, as shown on the map. Be sure to park in the eastern portion in the spaces designated visitor and not in the spots designated athletic parking as you will be ticketed.
Lewis Library is a 5-10 minute walk from lot 21. It is probably fastest to walk, but there is also a University shuttle that leaves lot 21 approximately every 15 minutes
(http://www.princeton.edu/transportation/EastLineFall11.pdf). You should get off the shuttle at the Frist/Guyot stop and Lewis Library will be right in front of you on the right side of the road (see map).