A limited amount of publically available documentation is available for the lwIP project. Some of it has been incorporated into this manual. The following lists useful documentation known about at the time of writing:
Adam Dunkel's Master's Thesis - the original description of lwIP design and operation, but now somewhat outdated.
Report by Adam Dunkel into the design and implementation of lwIP, including a sequential API reference, and example code. Largely still applicable to current lwIP, albeit incomplete. Available in PDF and compressed postscript (.ps.gz). A copy of the PDF version may be found in the doc/ subdirectory of the lwIP package in the eCosPro source repository (packages/net/lwip_tcpip/VERSION/doc/dunkels-lwip.pdf relative to the base of the eCosPro installation).
Text description of the lwIP raw API. A copy of the version at time of writing may be found in the doc/ subdirectory of the lwIP package in the eCosPro source repository (packages/net/lwip_tcpip/VERSION/doc/rawapi.txt relative to the base of the eCosPro installation).
Text description of the sys_arch porting abstraction layer. A copy of the version at time of writing may be found in the doc/ subdirectory of the lwIP package in the eCosPro source repository (packages/net/lwip_tcpip/VERSION/doc/sys_arch.txt relative to the base of the eCosPro installation).