Modern Optare Solos (Euro 5 and Euro 6) are heavily reliant on CAN bus communication. The workshop manual includes proprietary fault code tables and troubleshooting trees that generic OBD scanners cannot interpret. Without the manual, a "Transmission CAN timeout" code is just a string of numbers.
It is crucial to understand that there is rarely a single "book" that covers every aspect of an Optare Solo. Because Optare (now Switch Mobility) bodies are fitted to various chassis configurations and powered by different engines, the "Workshop Manual" is typically a collection of documents: optare solo workshop manual
While you will need the Mercedes or MAN engine manual for internal engine work, the Optare manual covers the installation of these engines. Modern Optare Solos (Euro 5 and Euro 6)