Well - the 6th edition rulebook is the place to start - although keep your eyes on Chaosium's website - 7th is not far away.
One of the great things (for an older player) is how backwards compatible Call stuff is - they system hasn't changed much at all over the years - sanity has been slightly tweaked, some skills added, some changed - but for the most part you can pick up a scenario form 1988 and run it with little or no modification.
I guess the biggest change has been thematic - over the years, the game hasn't gotten away form the epic, globe trotting, Indian Jones with horror type story telling and evolved into a more psychosocial and darker kind of horror. take a look at 2006's campaign book Tatters of the King and compare it to masks of Nyarlathotep and the difference will be readily notable.
Campaigns were never my favorite part of the game though anyway - though they appealed to a lot of role players. as a keeper I always found it difficult to manage - if you saw the PCs die time after time, there was little continuity to running a series of linked adventures - or you would be forced to strain credulity by bringing new characters in with links to the initial deceased investigators. But if you fudged the rolls to keep them alive - the horror mood of the game became harder to sustain as there was little fear of death. so for my money - the New England sourcebooks - the Lovecraft Country stuff were and are the best place to start and to build a campaign
thus I would recommend The Complete Arkham Unveiled form Chaosium and a couple of recent sourcebooks form a very good TPP - Miskatonic River Press. The books I reference are New Tales form the Miskatonic Valley and More Adventures in Arkham Country - featuring several new short scenarios in and around "Lovecraft Country" in new England.
