Most, but not all of his books are available for free around the web. The Internet Archive lists 84, some are multiple uploads of the same title, The Librum Online Library has a huge number of titles though not all free and there are many other sites around the web, such as Google Books, that have his books available.
In addition many modern publishers have reprinted many of his books and they are available on line and in bookstores. Lindsay Publications and 10 Speed Press are 2 of the reprinters among others.
By far, for me, the best experience is finding the originals in used book stores. I only have a dozen of his books in paper most original and a few reprints.
Below is a picture of an original 5 volume set of "Cassell's Cyclopaedia Of Mechanics" edited by Paul Hasluck and two reprints "Metal Working" reprinted by Lindsay Publications and "The Handyman's Book" By 10 Speed Press.
Volume 1 of the 5 Vol. set. Excellent shape considering it was published in 1903.
Lindsay's beautiful hard cover reprint of "Metal Working"