The NICE guidelines are more aimed at general practice but give a useful reference point for how to interpret investigations (osmolality ect.). Also has helpful list of medications that can cause a drop in sodium, alongside the more common things that can cause it.
The FOAM Resources
Some of the finest free to access resources out there
Easy to follow run through various causes and treatment. If you read one thing only this is the one.
Great, simple guide on what to do with unwell people with low sodium.
Summary of cerebral salt wasting syndrome and to SIADH.