For those researching Spanish heraldry, coat of arms, crests and such, this document explains alot. There are numerous books on heraldry. Some are theoretical studies on the concept of heraldry, the notions of shields, their interpretation, their symbols and terminology, which is already fixed and is typical of this area. Others are codes of shields, with explanation of its components, which contains sources studied or families who own them.
Keep in mind that many genealogy books include extensive information on heraldry. So many sources cited in the study of genealogy also serve to begin in heraldry.
To become familiar with these books, nothing better to start with the text of José Antonio Vivar del Riego.