These three books are the holy grail of background reading and fluff on the Imperial Guard. Now out of stock so they'll be harder to find but definitely worth picking up if you can find on online
Tactica Imperialis
[url]http://www.blacklibrary.com/all-products/Tactica-Imperialis.html[/URL]
With numerous detailed maps, comprehensive description of four Imperial campaigns and illustrated throughout, Tactica Imperialis examines in detail the tactics of the Imperial Guard and how they are deployed for war against a variety of foes.
The Imperial Infantryman's Uplifting Primer
[url]http://www.blacklibrary.com/all-products/Imperial-Infantrymans-Uplifting-Primer.html[/URL]
[url]http://www.blacklibrary.com/Warhammer-40000/Imperial-Infantrymans-Uplifting-Primer-Damocles-Gulf-Edition.html[/URL]
The definitive handbook for the loyal soldiers of the Imperial Guard.Bursting with a wealth of information describing tactics, combat,enemies, prayers and much more.
Imperial Munitorum Manual
[url]http://www.blacklibrary.com/Warhammer-40000/Imperial-Munitorum-Manual-The.html[/URL]
In the style of a military weapons catalogue, it features over sixty entries on weapons, kit and equipment utilised by the imperial guard.
Looking into this right now!
I am also looking into forge worlds Imperial Armor books but that will be on a different paycheck.