Ryandal…

At this late date, I would do research on-line and wait until the US to pick up the EZ66 Guide.  It's routinely stocked at the Mid Point Cafe in Adrian TX and Afton Station in Afton OK.  You'll want to hit both anyhow.  Price should be around $20-25.

For others, the book is available from Oklahoma-based author Jerry McClanahan through his website: https://www.mcjerry66.com.  It's valuable both in researching/planning your trip, and while on the road.  If you order it instead through a bookstore, the ISBN Number is 978-0-9709951-6-2.  This is for the current (2nd) Edition.

Some states have marked the route of 66.  Some have not.  Some signs get swiped–and missing just one will really throw you off.  So it's a good idea to have the EZ Guide with you. 

Flagstaff AZ and Santa Fe NM (on an earlier routing of 66) are both at 7000 feet (2135 meters) elevation.  Cold and snow should be expected in December.  In fact anywhere on your planned 66 trip might have snow–it just depends on the year.  The US government site https://www.weather.gov offers information on typical winters in cities en route.

The Blue Swallow Motel in Tucumcari NM has NOT been open year-round in the past–but it's under new ownership.  Be aware it only has 12 rooms.  Another solid Route 66 choice there is the Motel Safari (or Safari Motel).  And plenty of national chains are clustered out near I-40.

Some Route 66 attractions, such as Four Women on the Route in KS (home of the truck that inspired Tow Mater) are closed till spring.  The truck is outside and can be photographed near the restored pumps.  Perhaps you'll get lucky and have a light dusting of snow!

Let me add one classic motel to Steve's previous list–it's the Rest Haven on 66 in Springfield MO.  This is an hour away from the Munger Moss in Lebanon MO, and you have I-44 as an option.  With a little planning you can stay at one…and photograph the glorious neon at BOTH.  The Rest Haven's sign remains on until after sunrise.  I've stayed at both in the past few months to do exactly that…rooms are okay and prices reasonable.

And in Joplin MO there's the non-functioning but still wonderfully-over-the-top 1958 sign for the Capri Motel.  This is not actually on 66–which goes through the heart of town–but out near I-44.