Start with the PDP-8 Small Computer Handbook
It reads like a novel (kind of), includes a nicely illustrated chapter on how production of the PDP-8 was organised, and most importantly, explains the PDP-8 from the ground up.
You can pick either the 67-68 edition from the PDP-8/I era, or the later 1972 edition that focuses on the 8/e and its simpler front panel, but is a bit more extensive.
The PDP-8/I System User's Guide provides a quick hands-on introduction to operating the PDP-8. The basic steps of loading and using the RIM & BIN Loaders with the early standard software are outlined here, and next to the depth of the Small Computer Handbook this is a nice practical way to get an overview of the older PDP-8 software - Focal, 4K Fortran, DDT-8 and the PAL-III assembler.
Whatever you do, print out the PDP-8 Pocket Reference Card. No shirt pocket is complete without it. A printable version (think shirt pocket) is here (link).
At some point, you need to make the jump into the mid-70s and leave paper tapes behind you. That means running the OS/8 operating system and working with its programming languages. The PDP-8/I would often run OS/8 from DECtapes, but SimH allows you to hook up disk drives - even the RX ones normally meant for the 8/e. Which is the format in which OS/8 is typically found on the web. Getting on the Air with OS/8 is probably a good document to start exploring OS/8.
The documents highlighted above are just my idea of where to start. One of the best libraries is the PDP8online archive. Browse it for everything else!
This bootable RK05 image has a wide collection of games on its RKB0: partition. Of particular note are:
Chess - CHECKMO II. Having a chess playing program was quite an unbelievable thing in the early '70s. Here is its author's page for more background.
PiDP-8/I: RECREATING THE PDP-8/I
Software & Documentation
The best parts of a PDP-8 are its documentation and the available software. Both are unique in a machine of this era. Most '60s computers only had some software tailored to particular tasks, which makes them no fun to use.
But the most outstanding thing about the PDP-8 is its documentation, from entry level introduction all the way to hardware principles of operation. More than anything else, that makes a PDP-8 worthwhile to own and study today.
CELEBRATING 50 YEARS OF PROGRAMMED DATA PROCESSING