Depends what part you are working on. Database design falls into two categories - logical design and physical design. Most books discuss logical design which is really about relational modelling. There are several to choose from on that front. However, many of the hard and important problems are in the "physical design" whch including picking indexes, memory tuning, materialized views etc.
Morgan Kaufmann publishers have just released a new book on physical database design covering how to design indexes, range and hash partitioning, materialized views, storage layout, RAID, warehouse design and other physical design areas. The book covers Oracle and several other major databases. Lots of diagrams and examples. It's actually a really good book. An excellent book for people who design and administer databases.
Available on Amazon and most other book sellers.
Physical Database Design: The Database Professional's Guide to Exploiting Indexes, Views, Storage, and More
Paperback, 448 pages, publication date: MAR-2007
ISBN-13: 978-0-12-369389-1
ISBN-10: 0-12-369389-6
Series: The Morgan Kaufmann Series in Data Management Systems
Link on Amazon:
http://www.amazon.com/Physical-Datab.../dp/0123693896
Link on the publisher's web site:
http://www.elsevier.com/wps/find/boo...on#description
There's a companion book that focuses on logical design. A classic, already in its 4th edition. If you search Amazon.com for "database lightstone" you'll see both books.