Digital Design of Signal Processing Systems: A Practical Approach
https://pt.b-ok.lat/book/1060353/42fada
1) "<=" | non-blocking and is performed on every positive edge of clock. these are evaluated in parallel so no guarantee of order. An example of this would be a register.
2) "assign = " | continual assignment to wire outside an always statement. value of LHS is updated when RHS changes.
3) " = " | blocking assignment, inside always statements enforces sequential order.