Learning How to USE found records

I have located some regular army records which may be those of my Great Grandfather. The records come from Ancestry.com: "U.S. Army, Register of Enlistments, 1798–1914." This individual (if it is indeed one person) first enlisted in 1829 and apparently took his final discharge in 1850. The records are reasonably consistent in physical description and the ages are also reasonably consistent, but I have no means of being sure that these 4 entries apply to the same person. The entries do show the Regiment and Company he served in (i.e., 7 Inf A). Did soldiers have numbers in those days? Is there some place I can find muster rolls for the 7th Infantry, Company A? Are the records different for the regular army than for volunteer units?

More importantly are there books, websites, blogs, and so on which will teach me how to research the records for this man? I would rather do my own work (I then understand it better). Just asking someone to find answers seems to be a lazy act on my part and very unfair to others! But I don't have even a vague notion as to where to turn for answers.

Thank you for any guidance that I can get.