LibreOffice Base Handbook An excellent guide to Base. The example database used in the book can also be downloaded from this link.
In Pictures Offers an excellent step-by-step picture orientated Base tutorial.
Spoken Tutorials Offer a large set Base tutorial videos.
SheepDog Guides Offers a graded sequence of detailed Base tutorials, some case studies, and some "How To" pages.
Martin Baker Has written a tutorial about building a relational database with Base.
Andrew Pitonyak Better known in the Open Office community for his macro programming skills, offers a Base tutorial, along with some intersting examples.