Basis is the core administration function for SAP, a specialist field suited to those who enjoy the day to day administration of a complex, enterprise ERP system. Basis administration is a full time task with dedicated Basis administrators branching off into Security and SAP account administration (system security best practices dictates a separate and dedicated security function) Solution Manager is used in newer versions for administration of SAP landscape and Basis admins require full knowledge of the Solution Manager tool and the tcodes associated to SAP administration.
This rough document was created to provide a general understanding of Basis functions and screens, not pretty but pretty good!
A great daily checklist of administration tasks for the admin of the SAP landscape
SAP Administration Workbench a 500 page SAP book, hard to find...
The SAP Basis 101 guide is great as it covers core daily admin tasks and has screen shots for added ease of use. Great for Basis admins who are starting out or as as a refresher "ready reckoner"
SAP basis and Security Administration is a quick guide and provides an overview of the subject.
mySAP.com reference guide 400 pages dedicated to mySAP.com
Transaction Codes On a Page
a great tcodes-on-a-page guide from connexin.net
SAP Tables on a Page
a list of core SAP tables-on-a-page