As a PHP programmer (primarily) I found the book easily accessible and very well written and I would recommend it to anyone that wants to start learning MongoDB. I would add though that if you want to use MongoDB in a production environment a lot more is needed.
The main difference between programming to connect to a MySQL and MongoDb is that 99% of the time MySQL is used in a single server configuration, whereas MongoDB is used 99% in a cluster configuration. The cluster may have replica sets and shards and may be geographically distributed. So questions of from where to read and where to write are not obvious. MongoDB abstracts a lot of this but knowing the implications of the programming decisions is fundamental in a production environment.
Several important topics such as write concerns are not explained in sufficient detail. MongoDB basic production set up is a replica set with 3 servers (one of which may not hold data). Journaling should be enabled as well in production. All those characteristics make the system much more complex since data will not be written at the same time in all the servers.
So my recommendation would be to start with MongoDB and PHP and stop there if you just want to understand the document based model and the basics of programming in MongoDB with PHP.
Then continue with MongoDB the definitive guide if you want more details or want to set up a production environment.
All this is not to say the MongoDB and PHP is a bad book, on the contrary, it is an excellent introduction to MongoDB and I recommend it to anyone that wants to start learning this database system.