See also: Common Lisp links
If you're asking why lisp?— or If Lisp Is So Great, why don't more people use it?— I offer my personal answer and opinions, direct you to the back cover of Practical Common Lisp, a well respected study of languages and of course, Paul Graham's much discussed 2002 essay.
If you question Lisp's speed or power, these have long been addressed. As with anything, the more you know (and can get out of your own way), the better you become. Books mentioned below can help you get there.
Peter does an excellent job presenting Lisp for contemporary tasks and offers appropriate comparisons to Python, Perl, Java, C++ and other languages.
This is by far the best book, especially if you've already learned the basics in college or from an embedded version such as Emacs Lisp or AutoCAD's AutoLisp.
Peter Seibel has performed a wonderful service by making his book free to read on his website.
The last 100 or so pages was the closest thing to an O'Reilly pocket reference until Common Lisp Quick Reference arrived.
While I highly recommend his essays, this makes for a better second book by today's approach to learning languages. Once beyond the basics, it's beneficial having more than one source of information.
After he releases Arc, we can probably expect a new book, one hopefully as much a pleasure to read as Hackers & Painters.
It's still very much contemporary, despite predating the final ANSI spec of the following year. In fact, it comes highly recommended by seasoned Lisp programmers as the book for learning advanced macro wizardry.
After doing the occasional search for specific content and finally reading this fine book cover-to-cover, only now do I have a profound appreciation for it. However, I still place this as a third book to get for when you're ready.
Paul Graham clearly and gracefully explains things like what all the fuss about "functional programming" really means within context as well as revealing idiomatic usage, so it's like having a Lisp hacker at your side!
This is truly a gift to the community!
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History & Standards
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PS – While you can use it as merely a substitute for just about any language, it's best to learn the Lisp idioms for efficiency, readability and reuse. As someone who knew Perl and Java, was able to adopt Python and start writing production code after a day or two, you may find it takes a year to reach an equivalent level of proficiency with Common Lisp.
The learning curve can be fierce, but you're probably learning Lisp, Emacs, SLIME, how to interpret the HyperSpec and which libraries to use, all at the same time.
There's no magic pill to bypass the learning curve; there's no need to modernize Lisp's syntax; there's a strong and valid reason why libraries standard in other languages are effectively rewritten by each Lisp hacker; and when you're about one year into progressively deeper Lisp development, you'll understand, appreciate and relish the reasons why! That's a good place to be, and this page is to help you get there.