First off, I happily admit that my rating may be a bit too harsh, because I had very high expectations for this one.

Unfortunately, it was a big dissapointment. The premise was so fun, but somehow the end result is boring, predictable yet left me with a gazillion unanswered questions about plot, world-building and character motivation. There were also A Lot of plot points, sometimes major, that strained credibility, at least to me.

A sampling:

Why are there not more places like Agloe? How about fantasy maps? Why are maps not always a perfect representation of the world, since they change the world to look like them? Why is a single copy of a map enough to make a room appear in the museum, but is Tam going on about changing Agloe being possible only because she has the only copy of a map of Agloe. Why did Wally kill Irene? What exactly was Wally trying to accomplish with keeeping Agloe a secret? What made him so obsessive? 
Some of the bigger ones points of incredulity:
- Nell got fired and 'banished' from academia over a different initial assesment about whether a few maps were genuine or fraudulent. I know this was for Plot Reasons, but surely there could have been a more convincing conflict. 
Irene organises a full on gala in three days.
- The NYPL has no budget (his is a minor plot point), but Nell can have a job, any job she likes says Irene
- The Dreamers Atlas 1.0 is essentially an art project. Which is great and all that, but how are 7 (!!!!!) postdocs getting funded for an art project from a cartography department? And how is that project going to 'revolutionize cartography'? 
- The whole 'hide Tam in Agloe until it's "safe"' thing was completely unconvincing to me. I did not feel the danger

I would have DNF'd but I kind of stared hate-reading it...