In recent years FamilySearch has added two essential Maine Civil War resources to its website: the Civil War Soldiers and Sailors Card Index, 1861-1865, and a collection of images of enlistment and muster rolls for Maine regiments. The former has been indexed and may be searched here. The latter can only be browsed. By using the first resource as a finding aid for the second, one can locate and view images of the original records, which may include information found nowhere else.
A small tintype portrait of Maine Civil War soldier Jefferson Wentworth is now up for auction on eBay. Let's use him as our case study.
Searching for his name at FamilySearch gives us one good match. Clicking through to view the image of his index card, we find a wealth of information. He was born and resided in Embden, and was twenty-three when he enlisted in Company D, Thirteenth Maine Infantry Regiment, on November 9, 1861. He was mustered into the company at Augusta on December 10, for a term of three years. He was mustered out and honorably discharged at Augusta, January 6, 1865. We are told his occupation, marital status, height, complexion, the color of his eyes and hair. All valuable information, but all extracted from the original records.
In the lower right corner of the card are page references to the original rolls. We will use these to find records of Jefferson Wentworth.
The available records are listed here. We know that Jefferson served with the Thirteenth Maine, so we scroll down to find the roll that includes our first page reference: 3667. Here it is: "13th Infantry, enlistment rolls and Companies A-F page 3661-3671." Page numbers will appear at the top right corner of alternate pages. The "page" numbers are actually folio numbers, and refer both to the front and back of a leaf, so page 3667 may refer to either this image or the next. Scanning the first image, we find the signature of Jefferson Wentworth partway down the left-hand side of the page.
Many of these records are muster or descriptive rolls filled out by a clerk, but this is an enlistment roll. By comparing the signatures of the enlistees, we can see that each man signed his own name or made his mark.
The next page reference points to a descriptive roll, which gives us a piece of information that the index card omitted: Jefferson was enlisted in Norridgewock by J. M. Boardman. The company's muster-in roll (Jefferson is listed on the reverse) shows that he served in the regiment commanded by Maine's famous temperance advocate Neal Dow. A roll from November 1862 says that Jefferson was made a company cook — a position he would continue to hold in later rolls. In August 1864, J. Wentworth of Company D was reported to be "on duty in 30th Maine Regt. by S.O. No. 14." A history of the regiment explains that some members of the Thirteenth were that month "temporarily organized into two companies ... and attached to the 30th Maine" while the rest of the regiment was furloughed. The last roll on which Jefferson Wentworth's name appears confirms that he was mustered out in Augusta. Among other details, it tells us that he still had to travel thirty-five miles to arrive home.
These records from the Maine State Archives provide invaluable information on the state's soldiers, but, for family historians, more is always better. Look also at pension applications (Jefferson filed one in 1894, his widow in 1915) and pension payment cards (for the soldier and his widow). If you're lucky, Fold3 might have your soldier's pension file digitized. The 1890 Census of Union Veterans and Widows (which shows Jefferson living in Lawrence, Mass.) often provides details of a veteran's injuries or illnesses. The State Archives has municipal and regimental correspondence that might mention your soldier. Ancestry.com has draft registration records. Check also veterans home records, veterans cemetery records, and headstone records. Reports of Maine's adjutant general were published during the war, and offer both details on the service of soldiers and background information on the companies, batteries and regiments in which they served. Google Books has reports for 1861, 1862, 1863, and part of the report for 1864-65. There are also town and county histories, regimental histories and family histories to consult. (The Wentworth Genealogy places Jefferson within the family, and shows that he served with his younger brother.) Town records — some of which are online — may also provide clues to your soldier's service. As in many areas of genealogical research, the avenues are endless.
Update (Oct. 21, 2016): Maine Civil War Enlistment Papers, 1862-1865 are now available at FamilySearch.