PHP (recursive acronym for PHP; Hypertext preprocessor, and available at www.php.net ) is the most popular language on the internet at present, so it's a surprise that you dont see it that often. Wheras Micro$soft came up with ASP, PHP is "open source", which means its free and developed by lots and lots of people. Also, Apache and MySQL (the server and databsse comonly used with PHP) are also free, so you can see why it has such a big following.
I got into PHP at christmas, and think it's great - i think it's much easier to use tan ASP, and i think it has a lot more online support available. There's nothign i've discovered that one can do that the other cant, it's just a matter of preference.
As to your book, most good books on asp or php will cover SQL in them too, since they so often go togehter. I actually use PHP with IIS and SQL Server, and although most PHP books you get will teach you how to use it with MySQL, it's very similar to SQL server (and there are help forums around if you get stuck - www.phpbuilder.com/board is my fave at present). Most books (if not all) will cover everything, including forms and updating etc, but its a good idea to have a good handle on HTML first (since ASP and PHP generate dHTML dynamically - it's a good idea to know what you want to generate first!): personally i use an excellent PHP pne called Web develoipment with PHP and MySQL by Luke welling and laura thompson, and i'vejust used it to design £600 worth of intranet for a local company
ps oh and i use notepad to code all my php, and i did to do the asp i did too. and just to clarify those that dont know, SQL = structured query language is used to get data into and out of databases, php and asp is used to generate HTML. So tie them together, and you can use databases to genereate your websites for you 9yes i know its obvious, but there may be some people who dont know, like i once didnt!)