I also have Feis' book and yes, it's pretty good. It's not as thorough as Fishel's Secret War for the Union, but that one is strictly about the Eastern theater.
I think there may be more there to dig for in terms of Grant's intel. As Warren Grabau notes in his Ninety-Eight Days (IIRC), it rather surpasses belief that Grant made so many of his moves in the Vicksburg campaign without good intel, and it appears that Grant/his organization deliberately muddied the records to protect what may have been a fairly substantial informant network... some of the info attributed to "a helpful contraband" may in fact have been provided by local citizens of hidden pro-Union sentiment, it seems likely.
( I definitely enjoy that both Fishel and Feis discuss 'real' military intelligence as we would understand it today, rather than trotting out the oft-told/overtold tales of Belle Boyd and Rose O'Neal Greenhow...)
The questions left to be wondered about was how many Jewish people who were abused by US soldiers after Special Order 11, went south as information collectors. Railroad information was especially valuable to Grant. How many railroad managers were secret informants fpr the US? Cotton came out of the south, and money and food went in. How much information came out with the cotton?
Sheridan was known to run units of young soldiers who were often, "out of uniform" while in enemy territory. How many other US generals allowed that practice?
From a historical perspective, three types of intelligence operators are interesting.
Railroad men were connected across the front lines. They had an immense amount of knowledge about how the Confederate economy was surviving.
Commercial travellers, especially cotton buyers, were obvious suspects, from both sides.
Naval officers collected significant information about forts and vessels under construction. The US was rarely caught off guard by Confederate ironclads. They missed by a day in one instance, but with respect to New Orleans, celerity got the navy by the twin forts before the Confederate ironclads were operational.