The LD/50 of any substance is the amount - typically given in a milligrams of the substance per kilogram of the users weight (mg/Kg) - needed to cause death to 50% of users.
This of course is never tested on humans. Instead a drug is administered intravenously to rats.
The number is not very reliable for two major reasons: First of all, there is quite a difference between humans and rats. Humans have a completely different metabolism compared to our furry friends. This can either mean it takes more or less of a substance to reach LD/50, depending on the drug.
Secondly, many drugs are ingested orally. When something is absorbed by the GI track, it makes a pass through the liver. The liver breaks down a certain amount of the substance, usually into inactive chemicals (Although sometimes the liver breaks down an inactive chemical into an active one, such as with Codeine.
Wherever possible, this book includes the known human overdose threshold (Which is often a good deal below the LD/50), and/or a safe dosage range to stay clear of any dangers.