It's perfectly acceptable to quote someone else to illustrate a point and it can even be flattering to the person being quoted. It's not a bad thing for someone else to think that your words are so profound that no one else can possibly come up with the same meaning. However, if you use a lengthy quotation it is most advisable to request permission from the source or the dreaded plagiarism word rears its ugly head.
Really, though, do try to come up with your own way of expressing ideas. If you are writing a book, originality is the key to success. If the reader wants to know what someone else has to say on a subject, the reader might as well read the original.
In defense of quotations, at times they do lend credibility to your work as you are aligning yourself with a recognized authority. Just be careful about how many quotations you use and about the length of each quotation. And, be very diligent about the accuracy of the acknowledgment: as I told my client, the original source can usually afford a lawyer more than you can.
On that note, I am signing off for the Christmas season. Best wishes to all of you.
Denise
www.inktreemarketing.com
www.publishacookbook.com