18p means 18 picofarad, 4p7 means 4.7 picofarad and so on. The reason you measurements differ by around 35 picofarad is because you didn't subtract the capacitance in your meter and leads. If your meter has a "REL" button, press it before doing the measurement (with the lead tips not touching anything), and the meter will subtract its own capacitance. Otherwise, you'll have to do it manually: read the value that you get without touching anything, then subtract that from your measurements. 35pF is a pretty typical value for a meter's capacitance btw, but it can vary from meter to meter (I've seen >100pF), and may change depending on environmental factors such as humidity.
A 3 digit code like 103 usually means: "10", plus 3 zeroes, in pF, so 10000pF.
Are you sure the "C0B" isn't actually a "C0G"? That would be a code describing the temperature properties of the cap. More info and a list of such codes can be found on the wikipedia page for
ceramic capacitors.
Connecting caps in parallel is done frequently for a number of reasons, such as increasing the capacitance when you have a standard cap size on your BOM already, or influencing other properties such as ESR. The latter point is also why you sometimes find a 10uF and a 0.1uF cap paralleled.
Caps in series are more rare, because why do it when they're physically large devices and you'll end up with half the capacitance? The only reason I can think of off the top of my head is handling voltages above each individual cap's rating.