When originally published, the code from TLPI was licensed under the GNU Affero General Public License, version 3.
One reader commented that they are using one of my library functions as part of their everyday "toolbox" of handy pieces of code. As things currently stand, that would mean that if they redistributed any code that linked against that function or ran that code as part of a larger network application, then the complete source code of their application would also need to be licensed under the Affero GPL (i.e., all source code linked to my library functions would also need to be distributed).
That scenario wasn't really my intention (I'd overlooked the case of library functions in my code). So, I've now relicensed the library functions in TLPI under the GNU Lesser General Public License, version 3. The relicensing applies to all files in the lib/ directory of the source code distribution, and the changed licenses are already present in the latest source code tarball and online versions of the programs that went up in an update of the web site a couple of weeks ago.
Aside from the licensing changes, it's worth mentioning that since publication of TLPI, I've made a number of small fixes to various example programs. The fixes are listed in the CHANGES file that is distributed as part of the source code tarball.