How come the first tone in the character of 边 in the word 东边 is not written? I am reading my textbook and I notice that the the pinyin next to

东边

is written as

dōng​bian

Why is the first tone not present on the bian? The weird thing is that in my other textbook the pinyin translation of 东边 shows both the first tones present? 

Can someone explain to me this, I do not remember there being a rule like this?
In Chinese, there is the neutral tone (轻声) which does not have any tone marks.

In the word 东边, the "边" can be pronounced with a neutral tone.