So i recently got this book, i'm curious what other books go with this purposely.
It looks like you have a copy of the rulebook from the second version of the
D&D basic set. The direct companion to that is the
D&D expert set, as Corathon pointed out.
A brief release history of basic D&D might also help you...
First, in 1977, this
Basic Set was released.
Next, in 1981, this set was revised to make a new
Basic Set. You have the rulebook from this, also in that box were dice and a copy of B2:
The Keep on the Borderlands, so don't worry -- you aren't missing any rules if you have the book without the rest of the box. Also released in 1981 was the aforementioned
Expert Set, which is thus the direct follow-up to the book you have.
Beginning in 1983, those Basic/Expert sets were revised and three more boxed sets were added, making what is often referred to as the "BECMI" version of D&D, where BECMI stands for:
- Set 1:
Basic Rules
- Set 2:
Expert Rules
- Set 3:
Companion Rules
- Set 4:
Master Rules
- Set 5:
Immortal Rules
Depending on how keen you are to stick to
exactly the rules you currently have, you might want to consider upgrading to the above five boxed sets, since they cover a lot more ground than the first Basic/Expert sets, but the rules were revised, so won't match what you already have 100%.
Eventually, the first four boxed sets were revised and reworked to make the
Rules Cyclopedia. (The last boxed set, dealing with immortals became
Wrath of the Immortals.) Many people view the
Rules Cyclopedia as the jewel of the BECMI rule set, and it is hard to disagree. The
Cyclopedia packs a huge amount of content into one hardcover book, making perhaps the most complete set of D&D rules ever published in a single book.
To summarise:
1. If you want the direct follow-up to book you already have, look for this
Expert Set.
2. If you want one book that will let you run a "basic" D&D campaign, find a copy of the
Rules Cyclopedia.
3. If you like collecting early D&D material, you might want to spend some time browsing my
Mystara Collector's Guide, which lists all of the D&D material for that version of the rules.