The collecting of Civil War buttons is a category which can either be included in a general Civil War collection, a military collection, or a collection group all by itself.  It can further be broken down by excavated buttons (dug from the ground in camps, battleground areas, house sites, field camps,etc.) or non-excavated (found in sewing kits, old jewelry boxes, old houses, etc.).  The best standard reference for US military buttons in general was written by Alphaeus Albert many years ago--and is commonly called the "button Bible."  The real name is Record of American Military Uniform Buttons--and the Albert numbering system is still in use today.  The flaw in the Albert illustrated guide was that it did not accurately date when buttons were produced--in many cases, the face of a uniform brass button produced before or during the Civil War is identical to those produced well after the war years--and the latter are far less valuable to collectors.  The best book to assist with dating buttons was written by Bazelon and McGuinn and is a listing of manufacture dates of buttons based on the backmarks used--the small book is not heavily illustrated but does have most of the most common makers broken down photographically.  There are other books on the market, but if you have these two, you are covered 95% of the time.  Buttons are graded by the amount of original finish, often silver plate or gilt, how many dents are in the face, and the condition of the fastening shank.  Cuff buttons as a rule do not command the collector interest of coat size buttons and Confederate buttons are far more expensive than Union ones. 

The North South Trader Civil War Price Guide is a good indicator of the relative market value of all major collectible buttons from the Civil War years.  Illustrated below is a fine condition Confederate Staff Officer uniform coat button commonly called a "CS-5."