Neo4j is an open source product and the community edition is free
There is a paid for enterprise edition, which comes with additional enterprise level features (like HA and 24/7 support etc,) but it really depends on what you are ultimately going to use it for. Many people happily use the community edition for certain commercial purposes too. So I would argue its certainly worth a look at least, you can play for free and see if you like it! This page provides some more info on the different editions: http://neo4j.com/editions/. What features did you feel it was missing as I think Neo4j has quite a strong and comprehensive offering in this space?
Regarding your specific question about whether book looks to compare Neo4j against other graph databases. The short answer is no, it is a Neo4j specific "in action" book, and hence it tries to pack in as much as possible to try and help you get the most out of using Neo4j, which we happen to feel is the best graph database out there too, and why we wrote a book on it
Hope this helps, and hope you think about looking at Neo4j - I think you will find it fits most graph based use cases pretty well.
Everything you can do with Neo4j you can do it with OrientDB, and something more. OrientDB is really FREE for any usage, while Neo4j has a commercial license if you use it in commercial products. You can't use the Community Edition on your project, if your project is commercial.
About ArangoDB I can say that is not a GraphDB under the hood, it's a document database that executes JOIN on traversal like a Relational DBMS. About TitanDB I see it's more focused on high scalability and distributed processing, but take a look at the feature and community to get more information.
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