Sasines can be accessed in two forms. There are abridged registers from 1780 onwards up to 20th century for most Scottish Counties with some gaps, like no 'person' indexes or no 'place' indexes for some spans. You can look up the documents but the abstracts are very informative. They are quite handy if you are looking for a geographical location such as an estate or a farmhouse or a commercial unit because they give both owners and tenants or borrower and lender in the case of mortgages. They also give the superior of an estate as well as the undertenant.
They are in New Register House in Edinburgh. For earlier Sasines it is more touch and go as there are few indexes. You just have to wade through the abridgements around the time you are looking for then order sight of or copies of the documents. I have used them quite extensively for land history, particularly for a book I wrote on Eastern Renfrewshire called 'Eastwood District History and Heritage' 1989, and a work I co-authored with Stuart Nisbet 'Robert Allason and Greenbank'. We have both used them to study the histories of estates, cotton mills etc. They are useful for histories of land-owning families. Their main value to genealogists is for ancestors who were long term tenants or owners of property, and sometimes for lenders as they give kinship and inheritance information.
Unfortunately most of my ancestors don't figure in the Sasines. If you are lucky enough to have ancestors who did, they will make a difference.
Other sources: some major towns and cities have published topographic deeds. One of the most important examples are for Glasgow and surrounding areas where deeds were tested in Glasgow, such as Renwick's Glasgow Protocols. These are available in the Mitchell Library in Glasgow and I expect in the National Library of Scotland where other such collections are housed. Renwick's volumes are very well indexed and a very good source.
For both England and Scotland you can use the national public record office volumes (PRO England, SRO Scotland), for periods up to 17th century. In Scotland 15th to 16th century records occur in the Register of the Great Seal (Reg Mag Sig) and the Register of the Secret Seal (Reg Sig Sig - to some extent duplicates) which are Crown copies of land deeds with person and place indexes. Up to 17th century Register of the Privy Council contains court cases, pursuits, securities and victims of crime. In England there are Calendars of Inquisitions, Charter Rolls, Close Rolls etc for the medieval period, and Calendar of State Papers up to 17th century does more or less the same as its Scottish counterpart. These can be found in large town libraries.
Happy searching