17th Battalion, Mississippi Cavalry
(Also called Steede’s Battalion Mounted Partisan Rangers / the Lovell Rangers) It later became part of the 9th Regiment, Mississippi Cavalry (Miller's). A brief history is found at that link.
The 9th Mississippi Cavalry (Miller’s) (formerly 17th Battalion Mississippi Cavalry, aka Steede’s Battalion Mounted Partisan Rangers, & aka the Lovell Rangers) Internet site has a longer history taken from Dunbar Rowland's Military History of Mississippi, 1803-1898.
"Units of the Confederate States Army" by Joseph H. Crute, Jr. contains no history for this unit. 
Regiment Companies with the County of Origin
Men often enlisted in a company recruited in the counties where they lived though not always. After many battles, companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.
Companies Comprising the 17th Battalion Mississippi Cavalry:
Company A - (Steede’s Company) - raised in Jackson County
Company B - (Poitevent’s Company) - raised in Hancock County
Company C - (Evans’ Rangers) - raised in Harrison County
Company D - (Miller’s Company) - raised in Clarke County
Company E - (Mayers’ Company) - raised in Harrison County
- Beginning United States Civil War Research gives steps for finding information about a Civil War soldier. It covers the major records that should be used. Additional records are described in ‘Mississippi in the Civil War’ and ‘United States Civil War, 1861 to 1865’ (see below).
- National Park Service, The Civil War Soldiers and Sailors System, is searchable by soldier's name and state. It contains basic facts about soldiers on both sides of the Civil War, a list of regiments, descriptions of significant battles, sources of the information, and suggestions for where to find additional information.
- Mississippi in the Civil War describes many Confederate and Union sources, specifically for Mississippi, and how to find them.. These include compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- United States Civil War, 1861 to 1865 describes and explains United States and Confederate States records, rather than state records, and how to find them. These include veterans’ censuses, compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- Rowland, Dunbar, Military History of Mississippi, 1803-1898: taken from the Official and statistical register of the State of Mississippi, 1908 (Spartanburg, South Carolina: Reprint Company).
- Howell, H. Grady, For Dixie Land, I’ll Take My Stand!: A Muster Listing of All Known Mississippi Confederate Soldiers, Sailors, and Marines (Chickasaw Bayou Press, 1998).
- National Park Service, The Civil War Soldiers and Sailors System (accessed 11 January 2011)