The Spencer Manual of Arms.

For safety and efficient battlefield use all weapons officially adopted by the US military, even those in use today, have a "Manual of Arms". The Spencer was a bit of an exception. While officially adopted, there was no "official" manual of arms for the weapon.

Captain Greshom M. Barber of the 1st Battalion, Ohio Volunteer Sharpshooters created one that was distributed privately.  I've found a copy on the internet and you can download from it here.

Its an interesting read and worth considering if you are a member of a reenactment group deploying Spencers.

By the way, I've done some research and discovered that no one officially or otherwise wrote one for the Henry.  I believe that this is likely due to the fact the Henry was never officially adopted and was mostly used in a skirmish or specialty units.  

One of the more interesting units to employ the Henry was the bodyguard with Jeff Davis while he was trying to reach the western Confederacy after abandoning Richmond.  They were issued captured Henrys, most likely captured from the 1st D.C.