Software Project Survival Guide by Steve McConnell (ISBN: 1-57231-621-7) is a great place to start with project management methodologies if your work still has a bit of the Wild West flavor to it. McConnells work is all-around excellent...you can go wrong with anything he writes.
The Rational Unified Process, An Introduction by Philippe Kruchten (ISBN: 0-201-70710-1) or The Unified Modeling Language User Guide by Grady Booch, James Rumbaugh, and Ivar Jacobson (ISBN: 0-201-57168-4) talks about the Rational Unified Process, or RUP, an end-to-end methodology for software design. Its overkill for most FileMaker projects, but it can be mined for many useful insights that apply to smaller projects.
Extreme Programming Explained by Kent Beck (ISBN: 0-201-61641-6) and Extreme Programming in Practice by James Newkirk and Robert C. Martin (ISBN: 0-201-70937-6) contain many useful ideas to be had here, especially in the area of estimating time required to complete tasks, although FileMaker Pro database development isn completely compatible with extreme programming.
Code Complete by Steve McConnell (second edition) (ISBN: 0-73561-967-0), while the original book is over 10 years old, still is an extremely solid, fundamental book on good programming practices. This new edition is thoroughly updated. Its oriented toward structured languages like C and Java, but includes plenty of useful information for developers in any language. You might also try his more recent Rapid Development (ISBN: 1-55615-900-5) or Professional Software Development (ISBN: 0-32119-367-9).
Practical Software Requirements by Benjamin L. Kovitz (ISBN: 1-884777-59-7) is an extremely useful and readable book about developing requirements for software, including plenty of information useful to database developers. It contains some nice discussions of data modeling, among many other topics.
The Pragmatic Programmer: From Journeyman to Master by Andrew Hunt (ISBN: 0-20161-622-X) is a masterful selection of compact, easy-to-digest precepts about the craft of programming, and will benefit any software developer, regardless of the tools he uses.
Joel on Software by Joel Spolsky (ISBN: 1-59059-389-8) is a very assorted collection of incisive and often irreverent essays on various topics in software development. If you don feel like buying the book, the essays can be read from the archives of Spolskys website, www.joelonsoftware.com .