Users are advised to read :-
The documentation that comes with the PPP package.
The pppd and chat man pages; (use man chat and man pppd to explore these.)
The Linux Network Administration Guide (NAG); see The Network Administrators' Guide.
The Net-2/3 HOWTO; see Linux Networking-HOWTO.
Linux kernel documentation installed in /usr/src/linux/Documentation when you install the Linux source code.
The modem setup information page - see Modem Setup Information.
The excellent Unix/Linux books published by O'Reilly and Associates. See (O'Reilly and Associates On-Line Catalogue). If you are new to Unix/Linux, run (don't walk) to your nearest computer book shop and invest in a number of these immediately!
The PPP-FAQ maintained by Al Longyear, available from Linux PPP-FAQ. This contains a great deal of useful information in question/answer format that is very useful when working out why PPP is not working (properly).
The growing number of Linux books from various publishing houses and authors; You are actively encouraged to check the currency of these books. Linux development and distributions tend to evolve fairly rapidly, whilst the revision of books move (generally) much more slowly! Buying an excellent book (and there are many) that is now out of date will cause new users considerable confusion and frustration.
The documentation associated with the PPP tool(s) you are using The package specific documentation, usually easily available, is often the most useful when dealing with a specific tool.
The best general starting point for Linux documentation is The Linux Documentation Project Home Page. The HOWTO's tend to be revised reasonably regularly.
Whilst you can use this document to create your PPP link without reading any of these documents, you will have a far better understanding of what is going on if you do so! You will also be able to address problems yourself (or at least ask more intelligent questions on the comp.os.linux... newsgroups or Linux mailing lists).
These documents (as well as various others, including the relevant RFCs) provide additional and more detailed explanation than is possible in this HOWTO.
If you are connecting a LAN to the Internet using PPP, you will need to know a reasonable amount about TCP/IP networking. In addition to the documents above, you will find the O'Reilly books "TCP/IP Network Administration" and "Building Internet Firewalls" of considerable benefit!