What happened to Salesforce Technical Architect journey books?
Salesforce has assembled all certification related guides under this main link
Certification link: http://certification.salesforce.com/credentialoverview
Click on left side "Architects" menu for each respective certifications.
Now all the documents are pdf formats, so we can refer them at offline too.
Here is the Application Architect self-directed resource guide
Found some official statement from SF, buried deep in Certification FAQ:
Q: I cannot find the ebooks in Inkling. Where are they located?
A: We are in the process of migrating our ebooks over to Trailhead and they are currently located on our Certification website under the Architect section. Each PDF can be found with the associated certification that it references.