As I have always understood it, alkaline phosphatase is a marker enzyme whicn may be elevated with certain diseases of the bone and/or liver. In many cases, it serves as a non-specific tumor marker (as you have mentioned).
The following is an excerpt from the Merck Manual, Chapter 37, Section 4...
"Alkaline phosphatase in serum normally comes from the liver and bone and, during pregnancy, from the placenta. It is present in some tumors (eg, bronchogenic carcinoma which makes its own alkaline phosphatase). Bone growth causes an age-dependent rise in normal values, particularly in children < 2 yr and adolescents. Thereafter, alkaline phosphatase activity declines, reaching normal adult levels after a growth spurt during adolescence...
Alkaline phosphatase increases markedly in diseases that impair bile formation (cholestasis) and to a lesser extent in hepatocellular disease...
Isolated elevations (ie, other liver tests are normal) occur in granulomatous hepatitis or focal liver disease (eg, abscess, neoplastic infiltration, partial bile duct obstruction)...
In some nonhepatic malignancies without liver metastasis, the mechanism is obscure. For example, bronchogenic carcinoma may produce its own alkaline phosphatase; hypernephroma in 15% of cases induces nonspecific hepatitis as the presumed origin of the enzyme elevation. For Hodgkin's lymphoma, the cause of the isolated alkaline phosphatase elevation is unknown. Generally, an isolated alkaline phosphatase elevation in an otherwise asymptomatic elderly adult is not worth investigating. Most cases originate from the bone (eg, in Paget's disease)."