If you have a computer running UNIX, you can probably run MH -- and you can almost certainly run nmh. Ports to non-UNIX operating systems aren't too likely, I think, because MH and nmh take advantage of UNIX filesystem features like hard links that aren't available on other OSes. (If you know of ports, please click the link at the bottom of this page and tell me!)
You can get MH on the Internet or on tape. Someone who has a fair amount of experience installing UNIX software can probably get MH running on your system. The latest version (which this book covers) is numbered 6.8.4. MH 6.8.4 runs under Berkeley UNIX, System V, SunOS, Solaris, and other versions of UNIX. Some UNIX versions need patches -- so, if you have trouble building MH, check archives of the comp.mail.mh Usenet newsgroup or the mailing lists.
Unless you have a reason to get MH, though, I'd recommend the new version called nmh. nmh has much improved security and many bugs fixed, has more complete documentation and is much easier to install. It's evolving rapidly but is fairly stable; the latest version (as of this writing) is 1.0. It's available on the Internet.
What about the front-end interfaces to MH and nmh? The xmh program is part of the standard X distribution. The latest version runs under X Version 11 Release 6. You can also get MH-E and exmh from the Internet; the current versions are newer than the versions this book covers.
MH and nmh can get mail from another machine with POP, the Post Office Protocol. As of this writing, I don't know of any support for IMAP. IMAP archives are nowhere near as rich and flexible as the UNIX filesystem model that MH uses, and IMAP doesn't support all of the features that MH provides. So my guess is that full IMAP support won't happen soon. For the latest, check the newsgroup and mailing lists.