I recommend booking ahead of time, for two reasons.
(1) Knowing you have a room waiting for you is good if you will be arriving later in the afternoon - like on the LAST TRAIN from Bayonne.
(2) Spending two nights at SJPdP on arrival gives you one full day in the middle to get over jet lag, obtain last minute supplies, do souvenir shopping, and explore this beautiful, fascinating and historic small town.
I HIGHLY recommend booking into a hotel or inn and staying two nights. Start out after breakfast on the second morning.
The local cuisine is fabulous, and the evening PIlgrim Mass at 2000 is not to be missed.
Check in first thing the morning after arrival at the Pilgrim Office at #39 Rue de Citadelle, for the latest Napoleon Pass update, as well as any other information you might need. It's a great place to ask questions. Say 'hi' to Mona.
Also, the "best" scallop shells can be had here for a donation. They come from the local restaurants, as in the Middle Ages. Coquille San Jacques = Saint James Scallops. They are delicious too. Staff at the local Pilgrim Office drill the cord holes - so it is worth the added donation value to have the "real deal."
Also, check out Express Bourricot, next door, at #37 Rue de Citadelle. You can send a rollaboard-sized suitcase direct to Santiago for about €70. It arrives after a couple of days and goes into secure, locked storage at the Loop Inn, in the San Roque area of Santiago, about a 10-minute walk from the Cathedral. This Inn was formerly the Lasalle Hotel / Hostal.
I HIGHLY recommend this to get all the stuff you will not need until you arrive at Santiago, there, in advance of your arrival. You do not have to be staying at the Loop Inn to do this. However, I recall they might offer a lodging discount if you do.
Hope this helps,
Tom