The best books for strategic planning

1. On Competition, by Michael Porter: Porter's work is often criticised, and many other authors claim to have improved on it, but let us not forget that Porter remains the benchmark they are trying to beat. His style is, unfortunately, not the most gripping, but the insights he offers are essential grounding for any strategist.
2. The Balanced Scorecard, by Robert Kaplan and David Norton: While Porter lays out the grounding for strategy analysis, Kaplan and Norton set the benchmark for articulating strategy in a way that eliminates ambiguity and ensures it will get delivered. Once you've read The Balanced Scorecard, consider also its sequel, The Strategy-Focused Organisation, by the same authors.
3. Business Model Generation, by Alex Osterwalder: Oswerwalder tackles the tricky problem of how to ensure your strategy gets delivered in a functioning and fit for purpose organisation. Business Model Generation is a practical text which is elegant in its simplicity. If you're looking for more after reading this, consider also Operating Model Canvas from Van Haren Publishing.
4. Good Strategy, Bad Strategy, by Richard Rumelt: For me, the most interesting aspect of Rumelt's book is how clearly he spells out the mistakes organisations typically make in developing and executing strategy, and the consequences of doing so. A strategist needs to know not only what they should do, but also how to avoid these common mistakes.