You probably already know that JCo is the middleware of choice for building SAP-enabled components and applications...
in Java. I am still working on my JCo book, but since so many people have asked about the progress of the book, I have decided to make the current state of the book available as a PDF file. If you are interested in this file together with the Java sample programs it discusses, please send an email to email@example.com.
Users of JCo will also be interested to know that the latest version of JCo (1.1.03) will be available very soon (most likely this week, please check http://service.sap.com/connectors). This version includes numerous improvements and bug fixes. You should use 1.1.03 as soon as it becomes available. No changes to your existing applications are required.
ARAsoft has started to ship an extension to JCo, the "ARAsoft JCo Extension Library" which includes the "ARAsoft Java BAPI ObjectFactory" and adds many important capabilities to JCo in an easy-to-use fashion. Please send me an email if you want to obtain a trial version.