I've worked with iTextSharp for a long time and have come to appreciate its capabilities for PDF manipulation. I'm one of those people who likes to have a book to refer to, and there is an excellent book on iTextSharp that I highly recommend, "iText in Action", by Bruno Lowagie.

Up until version 4.1.6.0, iTextSharp was licensed under the LGPL/MPL dual license, which allowed for free use and redistribution in commercial applications without the need to open-source your own code. However, starting with version 5.x and later (now rebranded as iText 7), the licensing changed to the AGPL, which requires that any web application serving output produced by the library either be open-source or purchase a commercial license.

iTextSharp is now maintained by Apryse.