I came across a book written during the Great War that has some good tips for the surveillance operator. It introduces the essentials of spycraft of a bygone era, but it remains particularly relevant to the Investigator who conducts surveillance operations.
The attitude that espionage is a sport in which the players appreciate and honor each other is truly misplaced, but the author’s observations about how to look like you belong in a place and about the key elements of disguise are timeless. The author’s description of how he gained access to critical installations to make observations are as relevant today as the Balkans in the 1890’s.
My Adventures as a Spy, By Lt. Gen. Sir Robert Baden-Powell, is an excellent short read.