Chess: Middlegames by Polgar contains several thousand of these. Depending on the theme, some have tactical solutions but there are also many positions that have some concrete solution leading to a quiet += middlegame. Amazon.de has it for euro 22.95 ex shipping.
But really, I would just download the latest games from TWIC, use a pattern search in Scid and then pick some positions, if the goal is just to play against the computer. You seem to have very relaxed requirements, just browse through some games and pick a position :-)
I am collecting "basic" positions in a database, like K+B+N vs K mate, IQP, my own oppening pos, etc...... The aim of this database is to collect basic themes and every day choose two or three at ramdom an play them against computer. The idea is simple: learn by doing.
I know there is a book (GM-Ram) that cover 300 positions everybody should now, but althought I consider it a must-have books, 80% positions are about endgames.
I dont want to collect tactics exercises, but collect positions to play with and get a "feeling" about them, i.e. positions to defend, unbalanced positions, attacks, etc.. At the moment I have very few of them, but what I have I have learned a lot playing with them. Does anybodys knows if it is freely available to download a collection of this kind of positions? also any book suggesiton would be appreciated. Best regards. FS